Okay, put the convertdata.exe and the entity file(s) you need in some easy to reach folder, or just c:\ works well, you'll see why in a second..
Go to start > run, type cmd, hit enter to get that DOS window to open up.
You'll need to navigate to wherever you put the program by using the cd (change directory) command, so if you put it in c:\ you'd type
cd c:\
or
cd c:\whateverpathyouputitin
Once you're there, type convertdata.exe entity <filename> <filename> so for example
convertdata.exe entity AbilityRepairSelfLight.entity AbilityRepairSelfLightTEXT.entity
this *should* take the first file (in binary gibberish), convert it to text, and save it as the second file.