>if i try to 'active popup mymenu at x,y' i get an error "menu has not been defined with DEFINE MENU", although the command "do mymenu.mpr" runs that menu.

The last line of the MPR should be an "ACTIVATE MENU mymenu", add: "AT 20, 20" to the end of that line. If you want, add parameters to the MPR, the x and y positions. If passed, use them in the ACTIVATE MENU line, otherwise, just do the normal ACTIVATE MENU.