You can download the patch from the official Capcom website: http://www.capcom.com/support/sfxtpc_patch.html
You're looking for a PC patch to unlock all 108 characters in Street Fighter X Tekken! street fighter x tekken pc patch 108 character unlock link
Some users reported that Patch 1.02 (not 108 specifically, but close!) or later versions unlock all characters. You can try updating your game to the latest version. You can download the patch from the official