How to use ilMerge to merge dll’s into an exe file?

I tried:

C:\Windows\System32>ilmerge /target:winexe /out:c:\output.exe c:\MyProgram.exe c:\Microsoft.WindowsAPICodePack.dll

But got:

'ilmerge' is not recognized as an internal or external command, operable program or batch file.

What am I doing wrong? (And by the way, is there a better way to merge dll’s into an exe file?)

Thanks