I doubt it's your graphics card - I've got an Nvidia iMac, as well, and it works great. Your GeForce 9400 should be up to the task.
You say that the very first download/installation of Wizard 101 worked fine. Just to make sure we're on the same page:
You installed Crossover Games, first, and then downloaded the "Wizard 101.c4p" file from our website:
http://www.codeweavers.com/compatibility/browse/name/?app_id=4685
After downloading the "Wizard 101.c4p" file, you double-clicked on the file to open it in Crossover Games. You chose to "load" the C4P file, and then clicked on "install." Crossover then downloaded and installed Wizard 101, in addition to a few needed dependency packages such as Microsoft Visual C++ and the Flash 10 player.
The above method is the "standard" way to install Wizard 101 in Crossover Games. Is this what you did? If so, moving along -
How are you launching Wizard 101 now that it's installed? After the installation of any program in Crossover, you'll find a launcher item for that app in Crossover's Programs menu. In the case of Wizard 101, you'll find the launcher inside Programs>KingsIsle Entertainment>Wizard 101>Play Wizard 101. You're using this launcher, but it still won't go past the login screen?
After entering your login creds, does clicking on "login" even work? Does the client start to path its files, prior to showing you a clickable "play" button? At what point does the login screen prevent you from going any further?