If you download a Plugin of the Month or some other application that doesn't have its own installer you might find that when you get it "working" that it doesn't actually run. The key to get it working again most likely, assuming your paths and names are correct, is that you need to "Unblock" the .dll and .addin files so Windows 7 security will give in.
Right click on the file name > Choose Properties. If the Unblock button is there then security is telling you to "talk to the hand". Click Unblock and "she'll be right".
While attempting to get the new Wall Opening area I ran into this situation and this thread at RevitForum.org was useful background information. Gordon included part of a reply by Kean Walmsley with Autodesk.
Kean wrote:...snip "Many of the 2012 family of products use version 4 of the .NET Framework. .NET 4.0 implements slightly more stringent security than prior versions of the framework: if a DLL is suspected as having been downloaded from the web – as is clearly the case with our “Plugins of the Month” – the .NET 4.0 runtime will treat that DLL as if it has been loaded from a network share...snip