Hi,
Firstly, does the Windows-based disk image tool work OK and are you able to read/write Amiga disks?
Assuming all's OK, launch WinUAE and go to the "Misc" settings page (one up from the bottom of the list) and ensure "Catweasel" is ticked on the right-hand side of the "Advanced" settings area.
Finally, grab the current multidisk.device driver from
this page and follow the instructions supplied to install it within your WinUAE environment.
In theory, once you've done all that, you should be able to access your Catweasel as a "CAT:" device.
However, I've never been able to get it to read/write anything (it always says "not a DOS disk") and have to revert back to the Windows-based tool instead...
- Ali