Pretty straight forward: How does one say kaddish if he is in the middle of amidah? A few examples of this: If you come late; if you take a long time to say amidah.
One may not interrupt the amida to recite kaddish if one is in the middle of the amida. Rather, one should pause and silently (and attentively) listen to the person who is reciting the kaddish and fulfill his obligation thereby (Shulchan Aruch OC 104:7).
After the word yisbarach (after amein y'hei sh'meih rabba), one should continue with his amida and not listen to the rest of the kaddish (Mishna B'rura 104:27).
This approach should also be used for bar'chu and k'dusha (Mishna B'rura 104:26,28).
If one has already completed the final blessing of the amida (and preferably also the first yihyu l'ratzon), one may answer amein y'hei sh'meih rabba, etc., as well as amein to the sentence ending da'amiran b'alma (Mishna B'rura 122:1). (One may also respond to k'dusha, bar'chu, amein to the third and sixteenth blessings of the repetition of the amida, and the words modim anachnu Lach from the modim d'rabbanan).
If one has completed the final yihyu l'ratzon at the end of the amida but somehow didn't have a chance to take three steps back, he may respond amein to anything (Mishna B'rura 122:4).