Merriam-Webster is accessible through iFunbox, but it shows as "Dictionary". Surely I was expecting "Merriam-Webster".
Inside Merriam-Webster, you need to open the "MerriamWebster.app" folder. Inside it, you have to look for the Info.plist file. Then you can look at its content with Notepad (just drag Info.plist toward it).
Info.plist is an XML file, with several "key" and "string" pairs. One of the keys is "CFBundleURLSchemes", its string is "fb178450008855735iphonefree". It means that Merriam-Webster's URL scheme is "fb178450008855735iphonefree://", as odd it may sound. I've tested it and it worked.
There is a problem. I couldn't find a way of passing the word to the dictionary app. It means that you can create a menu button in Marvin for Merriam-Webster, but it just calls the app without showing the word. I think that you already could do this by multitasking. Unfortunately I can't find out how to pass arguments through the URL scheme. It should be documented by the developer. I tried to google it, and found this site with some URL schemes:
http://schemes.zwapp.com/?page=185 (still no way to pass arguments).
This is how far I could get, sorry.