This kind of problem happens to my PC with Windows Vista, where I have a copy of ZModeler 1.07b with both the default and the Maxoff's PKG filter (the filter file name renamed, so I can use it). After some using, for no apparent reason the Maxoff's one disappeared from the importing and exporting list. To solve this problem I have to delete and re-copy the folder from my other PC, where I don't have this problem.
Have two copies of ZModeler, especially if they're the same version, isn't a good idea, because they shares some files, like the configurations .ini one placed in the Windows folder.
Here's the Dummiesboy's topic about the same problem.