Choose:
File > Export > Desktop Publishing Formats > GEOTIFF/TIFF
Select Idrisi to geoTIFF/Tiff
Also export the palette as we want to keep the colors. Press OK.
Open and register the file with:
File > Open
Select your file and press OK. MapInfo recognizes the coordinates in the Geotif but not the projection, so we have to set it. Change the Category to: Non Earth and Category Members to: Non Earth (meters) (this is the same as a plane projection).