You can't put it in the BlitzMax/Mod folder itself, you'll need to make another folder within Mod. For example, BlitzMax/Mod/DM3D would be fine. Note that you will need to make a small change to the module to make it work, so I hope it comes with source. Assuming it does, open the main file ( which will be called *.bmx where * is the exact name of the module ( dreamotion3d? ) and look for the statement near the top which says something like
Module pub.dreamotion3d
Change that to wherever you moved it to. EG:
Module DM3D.dreamotion3d
and recompile it.
Also it's probably worth mentioning that module names and folders are best done without spaces and all in lower case. I recall problems cropping up when you use upper case letters or spaces in the name/folder. Numbers are ok though.
Module pub.dreamotion3d
Change that to wherever you moved it to. EG:
Module DM3D.dreamotion3d
and recompile it.
Also it's probably worth mentioning that module names and folders are best done without spaces and all in lower case. I recall problems cropping up when you use upper case letters or spaces in the name/folder. Numbers are ok though.