What you've done does indeed fix it, but the problem isn't what you think.
In fact, the =menu part is being ignored, since "menu" doesn't mean anything to the FOLDERCONTENT argument, but just having that extra word there is solving the problem...
This would've solved the problem in the same way:
Go FOLDERCONTENT=kittycats "C:\Program Files\Directory Opus\Menu\Internet"

It's down to a subtlety in the way Opus parses command arguments, combined with the Go command's PATH argument being implicit.
When Opus sees this:
Go FOLDERCONTENT "C:\Program Files\Directory Opus\Menu\Internet"
it will be interpreted as this:
Go FOLDERCONTENT="C:\Program Files\Directory Opus\Menu\Internet"
and that doesn't work because the Go command doesn't have a path to work on. The path has been swallowed up by the FOLDERCONTENT argument.
A very simple way to solve the problem is to reorder the arguments so that FOLDERCONTENT doesn't have anything after it to swallow:
Go "C:\Program Files\Directory Opus\Menu\Internet" FOLDERCONTENT
As I mentioned, the Go command's PATH argument is implicit. If there's something on the command line which isn't recognised as being part of another argument then it's assumed to be the path. (That way you can have simple commands like [b]Go C:[/b] without having to bother with [b]Go PATH C:[/b] or [b]Go PATH=C:[/b].)
Another way to fix the original command is to make it explicit that the path was for the PATH argument. Both of these work:
Go FOLDERCONTENT PATH "C:\Program Files\Directory Opus\Menu\Internet"
Go FOLDERCONTENT PATH="C:\Program Files\Directory Opus\Menu\Internet"
The = after PATH is still optional in these two cases. Even without the = sign there is no danger that Opus will interpret the command as Go FOLDERCONTENT=PATH ... or similar because Opus knows that the word "PATH" is an argument name and thus won't let FOLDERCONTENT swallow it.
However, in some rare situations the = is required because you want to specify a string (usually the name of something) that happens to be a valid argument name. For example, say you wanted to make a button which creates a folder called "INLINE". INLINE happens to be a valid argument for the CreateFolder command, so neither of these will do the job:
CreateFolder INLINE
CreateFolder NAME INLINE
You would have to use this instead:
CreateFolder NAME=INLINE