Given a youtube video URL, like:

http://www.youtube.com/watch?v=xQkBRD7HBvw

You append a ampersand sign (&) and then the time index. For example, to jump to 1 minute 53 seconds in the movie, do the following:

http://www.youtube.com/watch?v=xQkBRD7HBvw&t=1m53s

You need to simply type the time in the comments but you have to make sure that you leave a space before and after. For example, to post a comment that links to 1 minute and 32 seconds in a video, you would post a comment like:

 1:32 

Note the space before and after the time which is necessary for the link to succeed.