You are downloading the Stone Legion Modded Client .Exe Version. This one works on Windows Only. For Linux and Mac users, Please see the .Jar Version of the client.
Tips Installing and Running Modded Client:
In the Updater go to Options (Gear) and set the following:
Minimum Memory: 2G
Maximum Memory: 2G
Java Home Path: (You want to make sure you install and pick the java 64 Bit version. So if yours says for example C:\Program Files (86x) then you picked the wrong version. On 64 Bit Operating Systems It's just C:\Program Files for the 64 Bit version!
If you need Java 8 (x64) You can download it here!
Then save and you should be set! Post on our Support Forums for any issues or questions!