After loading the image file onto your Mac you will need to change the type and creator of the downloaded image file both to 'RK8E' before they will be usable. Dragging the image onto the "Change to DECpack Disk File" utility included in the PDP-8/E Simulator package will accomplish this.
DECtape images will be converted to 128 words per block format which will allow them to be mounted as an RK05 pack. They will not be bootable.