the same thing happened to me.
wat u have to do is make sure u downloaded the new client v0.55
then you will have a WinRar file in your maplestory folder if u saved it there.
DOwnload WinRar by just typing "WinRar download" into google. after u have winrar, double click the new GoDMS client file. It will be converted using the program, and the converted file will be in that program. Now you just copy the file in WinRar and paste into your maplestory folder. After doing that it worked perfectly.
if its not working, tell me where ur getting problems, ill try and help you out further =D