Log into your server. For purposes to s**ts and giggles, we'll say the theme you want to activate is called "Work you piece of crap" (Yes, I'm in a mood today LOL)
Now, surf to where you want to upload your theme, and upload. Your directory tree should look like so:
wordpress/wp-content/themes/work you piece of crap
now, click on the "work you piece of crap" folder. It *should* look like this inside of it:
But tell me, are you seeing this, instead?
/work you piece of crap
? If so, you've double-directoried your theme files. You need to open up the original folder and upload the subfolder.