Try these steps:

1. Click the Firefox button near the top-left of the screen.
2. Click Add-ons
3. Find BlockSite in the list
4. Click the Preferences button that appears in the same row.

I have FF 17.0.1 running. There's a TOOL button at the top of the browser with Add-0ns Manager on the list, BlockSite is not in that list. Perhaps I'm looking in the wrong place?

My mistake. Try these steps instead.

1. Click Tools at the top of the browser.
2. Click Add-ons
3. Find BlockSite in the list
4. Click the Preferences button that appears in the same row.

I re-added it but it's still not showing up. I'll keep fiddling

Do you mean you re-installed BlockSite? If you did, your next task would be to follow the steps in my last comment.