Thanks for the quick reply! If I get any further problems, I'll be sure to contact them directly.
I've managed to get it working, though I had to change a line of code in the Ruby file I mentioned. If anyone else has the same problems, go to C:\Program Files (x86)\Google\Google SketchUp 7\Plugins (or wherever your Plugins folder is for Sketchup) and open arplugin_trial.rb. Find the line that says
instdir = ENV['ARMEDIA_PLUGIN_TRIAL_DIR']
and change it to have the location you installed your ARMedia trial plugin contained in single quotes. For me, it ended up looking like this:
instdir = 'C:\Program Files (x86)\ARPlugin 1.3 (Trial Version)'