I worked through the problem with much help from someone who I saw had completed a wherigo with a Xoom. This is how we eventually got it working. Hope that it is of help to anyone with a Xoom.
Originally I had made a folder called wherigo and got the "Cannot crate Root directory, SDCard probably missing. Insert it into device and run again" message. I changed the folder to Whereyougo and the app opened but I got the "No Wherigo cartridge available. Add cartridge file (*.gwc) into /mnt/external1/WhereYouGo/ directory and start WherYouGo again!" message. I assumed that since it was originally looking for an SD card and that I got the message about the external1 folder that that was where it should be loaded. I was trying to figure out how to restructure the folders to make a /mnt/external1/WhereYouGo/ directory with no luck so I was about to give up when I reread your message and noticed that you had said something about making sure that it isn't on the SD Card also. I moved the whereyougo folder to the internal storage with the Camp_Saratoga cartridge (Pocket PC format) in the whereyougo folder. When I push start now the cartridge actually shows up and loads.
Originally I had made a folder called wherigo and got the "Cannot crate Root directory, SDCard probably missing. Insert it into device and run again" message. I changed the folder to Whereyougo and the app opened but I got the "No Wherigo cartridge available. Add cartridge file (*.gwc) into /mnt/external1/WhereYouGo/ directory and start WherYouGo again!" message. I assumed that since it was originally looking for an SD card and that I got the message about the external1 folder that that was where it should be loaded. I was trying to figure out how to restructure the folders to make a /mnt/external1/WhereYouGo/ directory with no luck so I was about to give up when I reread your message and noticed that you had said something about making sure that it isn't on the SD Card also. I moved the whereyougo folder to the internal storage with the Camp_Saratoga cartridge (Pocket PC format) in the whereyougo folder. When I push start now the cartridge actually shows up and loads.