That's why I said it's a little difficult to do

I do this:

1. Open PG
2. Click 'History'
3. Click 'Blocked' Tab
4. Scroll to bottom of list
5. Determine which entry is the result of the website you are trying to visit (usually in blue, will have your IP address as source, name may not tell you a lot unless you know a little something about who runs the website, etc.)
6. Right-click on entry and choose how long you would like PG to allow access to the site

I only do this for sites that I visit frequently, as this is a little tedious!

Good Luck