Missing codec h264 in After Effect CC (2015)

After Effects no longer encodes directly into H.264 (it didn't do a great job of it in the past). You can either render to an intermediate codec from AE or send your AE comp directly to the Adobe Media Encoder.

I almost always render directly from AE. This is because AE CC 2014 has multiprocessing. So, I will work in CC 2015 to enjoy the snappier interface and then open that project file in CC 2014 to render it with multiprocessing (since my system can utilize that feature really well in most projects).

Here's the Adobe Blog post about the removal: using Adobe Media Encoder to create H.264, MPEG-2, and WMV videos from After Effects

If you talk to a lot of the old-timers around here, they'll tell you that AE's h.264 encoder was buggy and didn't do a good job. They call its removal "good riddance". The AE team is pretty tiny, so putting their resources towards making AE work better at VFX, compositing, and motion graphics, and letting the Adobe Media Encoder team work on encoding things makes sense to me.