This may sound obvious but since it's a single mission, there is no campaign file. Thus, the mission can't be selected from Campaign room. It should be available in Tech Room -> Mission simulator -> Single missions.
The message "The currently active campaign cannot be found, please select another" will pop up often if you change mods and have a mod campaign selected. To get rid of this, simply go to campaign selection and pick one that is available. FS2 main campaign is always theoretically available for selection - although it won't be playable wih, say, FSPort enabled, because FSPort overrides FS2 table entries with FS1 tables...
Also, I would recommend putting it in ..Freespace2\FSPort\ directory, that way it'll be enabled when FSPort is selected. Since it is a FSPort mission after all, it requires that FSPort is selected as a mod as well.
You can of course have multiple mod directories enabled simultaneously (command line is -mod MOD1,MOD2,MOD3,...,MOD(N+1), although I don't know how many mod directories it can use - more than enough I would bet), and usually it indeed is better to install separate mods in separate directories, but in this case it's better to integrate the mission and the related files directly to the FSPort mod. I don't remember if this mission is packed to a VP or if there's a bunch of files, but in either case it should be relatively simple to install to FSPort directory.
Try that and tell what you find out.