If you haven't, then do so and then do exactly what it says. Hint: you must be logged in and running a shell interactive session and have changed directory into /mystic:
$ cd /mystic
before you can install and configure the bbs.
If you *must* start mystic from the desktop, then Exec="/mystic/mis -d" would appear to be what you need, but you must have run './install' and then configured mystic before trying to execute "mis -d" for the first time, or running it is likely to fail.
And anyway, why would you ever want to start something like mystic from the desktop when you can configure your RPi to start it automatically as part of the boot process and to stop it automatically when you shut the RPi down?
You'll probably need a copy of "Linux in a Nutshell" at some point, so you may as well get one now if you don't already have it or something similar, such as "Linux for Dummies" - a bad choice of title, since most of the "... for Dummies" books are good to have if you don't already have some knowledge of what an operating system does and how to configure and run it. The "Nutshell" book assumes you already know another operating system quite well, while the "Dummies" books don't assume much prior knowledge at all.