The forums use SSO (Single Sign-On), with your Topaz account, so as long as you are logged in to your My Account page, you should have access when you press the Login button.
It looks like you got logged in, as you created this post successfully
As for the multiple accounts, we currently have two sites, where it is possible to have two accounts:
Topaz Labs (Plugins) Site
Topaz Studio site
We realize this can be a bit confusing, but we're working on integrating everything right now. Soon!