You don't remap characters but raw keys.
On the us keyboard the : is on the same key as the ;
so hitting shift-; should give the :

See the readme for non-us keyboard layout usage.

The keymapper does not report the right keys in any layout.

For instance, typing "shift" + "." produces ">?"

Ok, so how do i get a key to output ":"

Klick on the ; key in the mapper, klick ADD, then hit the key.
Then shift-key (the one you hit) produces :

Great, now it produces <:-??,>>,;.

Forget it, lock thread.

Hehe delete the mapper file and start over 😀

Maybe you could explain what you want to do (as the : should
be accessible in dosbox for you).

I feel exactly the same about the mapper. I have never been able to figure out how it is supposed to work.

To undo the mapping, just delete the file mapper.txt in your DOSBox folder.

The mapper is there to swap physical keys. The problem comes in
when you don't keep that in mind, that is you want to swap characters.
This does not work, and never will. Games/dos progs work on the
lowest level with key scancodes, so that's the level where stuff can
be changed.
Another level, much higher, is the keyboard layout stuff, but this does
only work if the game supports layouts. If it just reads from port60
you can layout as much as you want, nothing will change.