I guess Im late to the party as usual.
Looking at the module code, I assume it is written for spasm. Mimas is kinda picky with moving addresses. RORG $ADDRESS should make it so that all following code will start at that address. This means that you should use RORG instead of ORG for Mimas. It also has tools for converting .asm files to its own appvar format (and back) for some easy copy and paste to and from the editor.