ALright, i'll try to be as clear as I can using iMovie 11 to get a watermark on your videos.
1st)Start up iMovie with the video you want to watermark.(If you finalized it, you'll have to re-do that after you watermark your video)
2nd)Where the video previews, there should be 5 boxes, a music note, a photograph, a T, a transition box, and a world. Click the T.
3rd) You'll see serval boxes, this will be where you want to watermark your video. I recommend "Lower","Upper".
4)After you decide which you want, drag that box over one of your video clips. (Should be at the start)
5)It will say "Title Text Here", replace that with whatever you want in the box. To change how it looks, on the Preview window, click Show Fonts.
6)Go back to where all your clips of the video are. You should see a blue box. Extend that box throughout your whole video (May take a while)
7)Enjoy!
Hope this helps!
or you enjoy my pain 
or better yet you think s/he' rocks
?
Only question remains and it's this: Will putting this iMovie watermark affect the intro and out of the video I'm editing. Thanks for your patience. I appreciate it <3
glad I could help!