Ensure it is not inside a ZIP or RAR archive. If it is, extract it so you have the raw .fifamod file ready. Run FIFAModManager.exe as an Administrator.
Locate your downloaded Messi 2011 FaceMod By ViP3eR.fifamod file. Download File Messi 2011 FaceMod By ViP3eR.fifamod
Click the button at the bottom of the manager window. Ensure it is not inside a ZIP or RAR archive