Yeah, I don't really understand theater mode either. I've tried (and failed) about four times to put together a "first day" montage type thing. However, I just found this on the COD website. I hope it helps. I'll be trying it tonight and I'll be sure to tell you if I find anything useful.
Find a segment you would like to capture, and select the RECORD option.
Once you have reached the end of your segment, select RECORD again to stop.
NOTE: Segments can be no longer than 30 seconds.
Select NAME SEGMENT > UPLOAD CLIP, then create the TITLE, DESCRIPTION, and TAG for the video clip
Select a slot in MY FILE SHARE to save the video clip.
Go to MY FILE SHARE and select the video clip.
Choose the SELECT FOR PLAYBACK option. That video clip will now be available to render.
From the Call of Duty: Black Ops THEATER, select RENDER CLIP.
After the video clip plays through, it will be uploaded to MY THEATER.
Watch the video clip in My Theater or on YouTube.