I just downloaded SADXLVL a while ago and set up everything in the SADX folder but after I open the "sonic.exe.ini" file, I select a level (it doesn't matter which one) and I get this error message:

Error: OverflowException
Arithmetic operation resulted in an overflow.
at SADXLVL.COL.COL..ctor(Byte[] file, UInt32 address, FileTypes type) in C:\Users\Mike\Documents\Visual Studio 2008\Projects\SAModel\SADXLVL\Form1.vb:line 0
at SADXLVL.COL..ctor(Byte[] file, UInt32 address, UInt32 Count, FileTypes type) in C:\Users\Mike\Documents\Visual Studio 2008\Projects\SAModel\SADXLVL\Form1.vb:line 3640
at SADXLVL.GEO..ctor(Byte[] file, UInt32 address, FileTypes type) in C:\Users\Mike\Documents\Visual Studio 2008\Projects\SAModel\SADXLVL\Form1.vb:line 3551
at SADXLVL.Form1.LoadLevel2() in C:\Users\Mike\Documents\Visual Studio 2008\Projects\SAModel\SADXLVL\Form1.vb:line 682

I thought this might have been caused by having set the wrong .exe version (I believe I have EU; it's the SoldOut Software version and site is .uk), so I changed it a few times, but still doesn't work with either one... Please help, and thanks in advance!

That version is not supported. You can get a supported EXE here (US version).

As MainMemory said this version isn't supported. I have some experience with other Sold Out products and they seem to mutilate a few of the games they release.

You should still be able to do character, texture and sound mods, though, but that's the only thing you can do with the Sold Out version.