I know after you play a level you can rate it, but im not sure how to tag it or is leaving a comment considered tagging it?

When you rate a level after playing it, it should allow you to pick a word that you think describes the level after rating it. This is a tag.

yeah, when you finish a community level and asks you to rate the level, you should see a bunch of word bubbles. just select one of the words and you will have tagged the level...