I think all versions are the same, but I can't remember so...
It should be rougly along the lines of Click the Tools...Options menu item, and switch to the privacy tab (if you don't have that one, then search them all until you see..) to list boxes appear, with contact names in them. One is the allowed list, the other is the Blocked. Click the contact you have blocked, and then click the button to move the contact into the allowed list. Your contact should be able to connect now.