In file explorer, by default, the file extensions are hidden. If you right click the file, and select "Properties" you should see some facts about the file such as size, and type of file. It should say it is an executable file (.exe) and the size should be about 1.17 GB for "Install Poser 11". If those two things are not true, the file may need to be re-downloaded.

Ah, just saw your post after I sent this one. Seems you are on your way. Good luck!