When macros security is set to ‘High’ Excel doesn’t load XLL without notifying the user that the XLL wasn’t loaded. A ‘Medium’ security setting permits the XLL to load.

Remove need for msvcrt80.dll by compiling your project with /MTd compiler switch.

Summary:
There are two common errors
1) nothing happens — this is almost always security settings
2) “unrecognizable format” — this is almost always missing dlls

Some people manage to avoid problem 2 by using /MTd to change the runtime library

alternate / expansion ideas at:
http://www.jensign.com/primes/index.html