Please download the latest version [v2.6.1] of the SketchUcation PluginStore RBZ from here:
http://sketchucation.com/pluginstore?pln=SketchUcationTools
Then use the Preferences > Extensions > Install... button as explained before...
and restart SketchUp to ensure the changes are set up properly...
Any luck with it now ?
The PluginStore dialog being blank is probably result of some Internet Explorer interaction or settings...
SketchUp's web-dialogs always use IE on PCs - even if you prefer another browser for normal web-use...
You should update IE to the latest version - because if you have say IE8 or 9 it can cause issues.
IE11 is the current / best version.
In 'About IE' also tick 'update automatically...'
It might also be a 'cache' issue.
In IE clear its cache - Ctrl+Shift+Delete - or via the Safety menu item.
Then reopen the PluginStore dialog - you might have a corrupted cache, so it could now work.
Also can you open the Ruby Console and type in this + <enter>
SCF::TEMPDIR
What do you get as the result ?
Any results ??