no you havnt forgotton anything i would think the problem is the slot being set as preinstalled it should have inserted itself over that slot but there you have it
Remove the new Card.
enter the extension Settings and delete all the extension numbers assigned to cards that are not installed then try to delete the preinstalled card .
if that doesnt work make a backup of the DSSYS file from the sdcard and open it from the desktop and remove the preinstalled card then upload the fixed DSSYS file to the SDCard.. using (Reset by the command )procudure reset and use skip when asked if you want to make a backup of the current SD Card. the system should start with the new Fixed DSSYS file.
then try to install the new DLC Card.

peter ( www.vpi.ie)