How to add a directory to my path that includes a colon and a space?
It won't work. You have several options:


Create an alias for bash to run the command.
Create a symbolic link to the executable from a directory in your PATH. (I like to have a directory ~/bin at the front of my path for such things.)
Create a symbolic link to /Applications/MIT\:GNU\ Scheme.app/Contents/Resources somewhere and put the link in your PATH.
Rename the application.