In case anyone is building Jace from source, here is a run-down for the new media manager:
When the emulator main window is active, press F1 to bring up the media manager. The old shortcuts for inserting disk images are no longer relevant.
From your OS file manager (explorer, nautilus, whatever is relevant to your OS), locate the disk image you want to use and drag it in to the media manager window. Right now it's kinda finicky, but if you drop it in the lower half of the window (where there will be a details section eventually) and drop the file then it should create an entry. Look in the left-side for the first letter of your disk and click the letter. You should see your disk image show up in the list.