Then you should be able to just slip your mini-disc into place, if the inner ring Mr. B mentioned is present in the drive tray. There are "adapters" (for want of a better word) that are like an O shape that fit around the mini-disc, basically making it into a full-sized disc that'll fit into any drive tray - I had one with a 3" CD single I had back in the day, but whether they still exist or not, I've no clue.
Is this a burned mini-disc, or a pressed one? The problem could lie with the disc itself, and not the drive tray.
EDIT: Nevermind, you posted as I was typing, glad you got it sorted.