WireframeSketcher releases prior to 4.0 are not digitally signed. The default setting of Mac OS X 10.8 (Mountain Lion) is not to allow you to download applications unless they have been signed with an Apple digital certificate. When opening an unsigned application you’ll see a warning that looks like this:

Gatekeeper Warning

To fix it, open your Downloads folder in Finder and then Ctrl+Click on WireframeSketcher app. Choose Open from the context menu and then choose Open again from the warning dialog:

Gatekeeper Open

You need to do these steps once, and then the app will open normally next time.

Another, less recommended, way to fix this issue is to go to System Preferences > Security & Privacy > General and set Allow applications downloaded from to Anywhere (this was the default behavior before Mac OS X 10.8).

Changing Gatekeeper settings

For more information see this article on lifehacker.com.