Chances are you do not have sufficient access-rights to add/change files/subfolders in your SketchUp Plugins folder...
By default Windows will probably have limited you to have Read-only access, because the Plugins folder is within the 'Program Files' folder 'tree'... BUT you will need Read/Write [FULL] rights to add/change its files/folder...
So... you need to find your Plugins folder's icon...***
Select it, right-click > context-menu > Properties > Security > Edit...
Ensure everyone/every-group listed [of course including yourself] have FULL rights to that folder...
You might have to have admin-rights to change this...
Ensure that these FULL rights 'trickle-down' to all of its contained files/subfolders...
Also check that the owner of the folder has similar rights, if not perhaps try changing its owner to be 'you' ?
***You are on a PC so depending on your OS your Plugins folder should be in
C:/Program Files/../Sketchup../
But copy+paste+<enter> this code into the Ruby Console if you are not completely sure...
Sketchup.find_support_file('Plugins')
it'll return somethings like...
C:/Program Files/Google/Google SketchUp 8/Plugins
Other related SketchUp folders that you might want to write to from time to time... include Components, Materials and Styles... which should also have FULL permissions set...
