D: I notice we have not talked about my birthday yet, even though we touched upon it a while ago.
"D= Describe the current situation. Tell the person exactly what you are reacting to. Stick to the facts."
Depending upon how neutral you wish to make this request, you might even let go of the context which you bring up. Forget that it was brought up a while ago and you have not since talked about it. Consider:
D: My birthday is coming up. And I enjoy spending time with you. I would like to enjoy spending time with you on my birthday if this is possible.
E: I feel sad about this. (I also feel disappointed - should I say that?). It would mean a lot to me to spend my birthday with you.
"E= Express your feelings and opinions about the situation. Assume that others cannot read your mind. Donâ€™t expect others to know how hard it is for you to ask directly for what you want."
Unless you want to specifically bring up the point that you feel sad that the topic of your birthday was brought up before but not since (which may make the other person feel defensive), you might try:
E: It would mean a lot to me to be able to spend time on my birthday with you.
A: Could you find a way to be with me for my bday?
"A= Assert yourself by asking for what you want or saying no clearly. Assume that others cannot read your mind. Donâ€™t expect others to know how hard it is for you to ask directly for what you want."
"be with me" is slightly ambiguous. You might consider a more specific request, ie. share a meal, and/or spend time doing something we both enjoy, and/or go on a hike with me, etc... If you make it a specific request, then there is less concern on the other party with coming up with an idea which may or may not be rejected.
R: If you can make it, I would feel happy, and it would make me feel wonderful. I would be happy to spend important dates of your life with you.
"R= Reinforce the reward to the person ahead of time. Tell the person the positive effects of getting what you want or need. Help the person feel good ahead of time for doing what you want."
I think it is generally a safe assumption that if you enjoy spending time with the other person, they should also enjoy spending time with you. However, your "reward" presumes that they do not have some other different plan they might like on their birthday (i.e., if they are more extrovert, they would want a large group gathering, etc..)
Depending on what specific activity you request, the reward can be related to that specific activity (i.e., great meal at such-and-such place/food they enjoy, or if a hike, then enjoy spending time in nature or exercising for fitness, etc...).
Generally though, "I would be happy to spend time with you whenever you would like it." would be a neutral mutually receptive reward. It opens up the offer to what ever days they might prefer, including (but not limited to) important dates on their life.
"M= Mindfully keep your focus on your objectives. Maintain your position. Donâ€™t be distracted."
My take on being "mindful" is to pay attention feelings this request may elicit unexpectedly from you. Such as interpreting any hesitation on their part as a sign of potential rejection which the real reason might be much more innocuous. The key is to pay attention to your feelings and not necessarily act on them, or let these feelings push you in a direction which is counterproductive to your goal.
"A= Appear Confident. Use a confident voice tone and physical manner; make good eye contact. No stammering, whispering, staring at the floor, retreating, saying â€śIâ€™m not sure,â€ť etc."
I see this part as very much tied to being mindful. You want to communicate as clearly as possible, and so you want to avoid expressing other mixed emotions which may confuse the other party, or may even perhaps trigger the other party's own set of insecurities what not. So it is best to "appear confident" even when you might not be feeling this way.
"N= Negotiate by being willing to give to get. Offer and ask for alternative solutions to the problem. What am I willing to â€śsettle forâ€ť or â€śgive upâ€ť in order to gain what I want in the situation?"
Are you willing to celebrate your birthday on another day if that is an arrangement that is offered? I don't know what is going on in the mind of the other person, but you might be able to speculate, and you should ask yourself, what kinds of compromises are you willing or unwilling to make?