The issue is that I get an error saying there are no slots available, but if I'm getting a premium download, that shouldn't be an issue (unless filesmonster is a big liar).

Otherwise go to the forum and report this:
* I'm using v.083u1 by Lukiz+ntoskrnl
* The plugin version is 1.0.3
* I get an error that says AJAX server response: no free download slots available

What do you mean with "I'm getting a premium download"?