How do you register a username on Isketch?

At present you don't. As simple as that.

For more info please contact info@isketch.net

Uhm.. wut?

Ikiriz, I told you in PM that you could search for threads about registering.

If you click here (http://www.isketchforum.net/help-f14.html) to show all the threads in the help forum you can then search for ones containing the word 'register' at the top-right.

The main navigation at the top also has a search feature you can use.