Okay, I'm probably being annoying now, so sorry.

I'm trying to create a menu, but it says the URL is not valid(I didn't put one in).

What type of menu, a tab or a sublink?

I'm trying to add a tab. :)

1) Create a tab. (enter any url to your board, like forum.php)
2) Create a menu for the tab.
3) Edit the tab and find the setting, "Target Menu" select the menu you just created, click on save.
4) Add links to the menu you created, these are what will show up as the dropdown items in the tab.

So say if I wanted to add a link to the Mod forum, where would the link be placed?

Not sure what you mean by Mod Forum?

Moderator Forum. :)

That is not how it works, you add the link to the subnavbar, or do it in a droptab. There is no way in the default script to add it to a specific forum.

So it has to be something built in to vBulletin? I couldn't add a forum link into the menu?

You can add a link to any page on your site via the Navigation Manager, or to any place on the web for that matter.