Thereare other ways to patch the main TRE so the game will use the external files. One is located inside my GUI for HCl's mission compiler (you don't have to code missions to use it BTW). Look for 'WinConsole for WCP mission compiler v3.0' in the files section of my site.
If you want to play SO missions, the easiest is to use Locke's simulator patch, then just replace the \mission\sX.mis file with the mission you want to play.
If you want to play WCP missions, you have to patch the game first. You can use HCl's patch to use mission slot #0 (\mission\s0.mis) or my WinConsole to patch all the slots you want (GUI, very easy). Then you'll have to create a "mission" subdirectory in WCP's main directory (if it doesn't exist) and put the mission you want to play there (renamed sX.mis, where X stands for the mission slot number that you patched).