First of all make sure that the MD5/SHA1 checksum for the downloaded .dmg file matches its published value.
Then make sure you have your License Key and then follow the directions in: 
Manually uninstalling VMware Fusion (1017838)
After manually uninstalling VMware Fusion then reboot the Mac.
Now try installing either VMware Fusion 5 if you don't want to try VMware Fusion 6 or VMware Fusion 6 if the MD5/SHA1 checksum for the downloaded .dmg file matches its published value.