What do you mean 'cannot assign'. What do you see? Are the buttons greyed out? Do you get an error message somewhere?
The buttons are greyed out
Can you create a new map file. Does it work that way?
Do you open all map files or the map file specific?
Ok, my own ineptitude. I kept the default at "All Map Files" thinking this was an actual selection. In fact I had to pick an actual file from the drop-down list. Thank you so much for trying to help!