Thank you very kindly!
That reply set me on the right path, although I did run in to some other issues I was able to resolve.
For the sake of posterity, I'll elaborate a little.
You were indeed right that I needed to link in the 1.6.1.jar file. I was uncertain if that was the minecraft file I needed, simply because the name was not particularly clear (One might deduce it's nature, but it is just obscure enough to be uncertain).
Because I wasn't sure, I instead downloaded and linked the Minecraft.jar for version 1.6.1 separately.
Once I'd removed my original link, I linked the 1.6.1.jar file in the .../mcp/jars/versions/1.6.1 folder.
However, once the project rebuilt, 22 errors were generated. I took note of the errors that mentioned anything to do with imports and isolated the issue was that I had not yet linked in joptsimple.
Fixing this was as simple as linking in jopt-simple-4.5.jar from /mcp/jars/libraries/net/sf/jopt-simple/jopt-simple/4.5/
Again, thanks for the help. It does seem simple to me now, although I wouldn't likely have thought the jar I was using would function, but would not be correct (