How do I comment on the Scratch Wiki? It seems really complicated.

It's not too complicated.

First, see http://wiki.scratch.mit.edu/wiki/Help:Welcome/4.

Basically, you indent the amount you need to to reply using colons, write your message, and then leave 4 tildes (~~~~) to sign your signature and a timestamp.

So, a normal chain where people were simply replying to the post above them would look like this:

Hello. I'm User1. ~~~~
:Hello. I'm User2, replying to User1. ~~~~
::Hello. I'm User3, replying to User2. ~~~~

You normally indent 1 more than the amount the person you're replying to did.

After six or seven indents, the next reply is outdented (See the linked page).

Oh, and you have to push “Edit” on a talk page you are replying to in order to do any of this.