I'm not sure why you get the error, is there anything under Tools -> Log?

I generally stick to the command line when installing things.

Try installing "becomeroot" from the application manager, then open up an X terminal, type "sudo gainroot", and finally change to the directory you saved the .deb file to (example: "cd /media/mmc2/downloads"), and type "dpkg -i galculator_1.3.1-1_armel.deb"
(without all of those double quotes)