I have this card but its been ages since I changed the memory configuration in it. the card has its own software and doesn't autoconfigure the RAM. you have to set the RAM amount and in the startup sequence put the program that runs to add the RAM to the available pool on the Amiga. SetXA and autoXA or somesuch. this page has the disk images you need for the programs you need to set the RAM amount and get the Amiga to use it:
http://amiga.resource.cx/exp/mbx1230I don't think I see anything about this software in the manual there but you definately need it and the software is pretty self explanatory. Might even be a manual or readme on the disk.
