Theme(s) Not Uploading and Plugin Manager Not Working

[Soulcist]

I have downloaded a various amount of themes so that I can add a theme to my board. None of these themes will upload, it always tells me this: http://puu.sh/5kHVG.png

Not a single one of these themes that I've attempted to upload has worked whatsoever and I doubt none of them wouldn't work but here and there.

So at this point I'm not sure what I'm doing wrong. And the URL (in case it's needed) is http://bronzeage.net84.net/index.php (I'm messing around with MyBB on a free host before I decide to use a real one and what not so I don't have to pay).

As for my plugin manager, it doesn't seem to be working either, I'm getting this error message: http://puu.sh/5kIwr.png

[.m.]

see themes installation guidance
if we can't browse plugins section from admin panel for some reason then we have to visit Mods section through regular
browser, find and download required plugins and install them manually (replies here might help for plugins installation)

[Soulcist]

Thanks a bunch for the help, this makes much more sense now (I'm new to this software).