Need help decompiling

[6x9]

Attempting to decompile this program. http://www.mpgh.net/forum/showthread.php?t=1054854( ORape 3.1 ROTMG Hacked client. ) What I am attempting to do is decompile this program and change the programs outdated packets with updated packets. The problem I am having is everytime I attempt to decompile it using http://www.dede.com-about.com/( DeDe 3.50.02's Free decompiler. ) I get this this error.

https://imgur.com/rcpda3X

Does anyone know what I am doing wrong? Should I know what language it's written in? Should I be going a whole different route with this?