ipkg -d sd package_name.ipk
is an option. I've not used the GUI package manger for a while, but the drop down list of destinations always used to sit in the top right hand corner.
Simon
P.S. If these don't work, look in /etc/ipkg.conf and check the 'dest' lines (make sure there is a 'dest sd /mnt/card' (hopefully that's the right way round ;-))