This worked for my setup on Windows 7,
-Install the latest Drivers and Apps Suite from Prusa's website.
-Under Start Menu, Open 'Windows Control Panel' then 'Device Manager'.
-IF it is listed as 'Unknown Device', then right click and uninstall it, unplug MK3, reboot computer and replug MK3. Should now show up as Original Prusa i3 MK3 with a yellow exclamation triangle.
-It should be listed as Original Prusa i3 MK3 with a yellow exclamation triangle, then...
-Right click it and select 'Update Driver Software'
-Select 'Browse my computer for driver software'
-Navigate to Prusa3d Drivers Install path (defaults to C:\Program Files\Prusa3D\Rambo)
If successful the printer should now be listed in the Device Manager under Ports and should now be seen by all programs (slic3r, Pronterface, etc)