There is at the moment no official x-plane 11 support from PMDG for their DC6 product, only X-plane 10.
However there is a workaround to make it work in Xplane 11.
Step 1: Download and install the DC-6 Base package XPL.exe from the PMDG website.
Step 2: Intsall into Xplane 11, for example:
Step 3: fix the reversed rudder animation.
Go to: X-Plane 11\Aircraft\PMDG\DC-6\DC-6A\objects
Open the file pmdg_dc6b_tail.OBJ with notepad
Find the anim_rotate key values near the bottom of the file, and adjust as below
ANIM_trans -0.0007 4.2285 16.2842 -0.0007 4.2285 16.2842 0.0 0.0
ANIM_rotate 0.0166 -0.0001 0.0008 1.9008 1.9008 0.0 0.0
ANIM_rotate_begin 0.0 1.0 0.0 com/fyc/dc6_958_obj
ANIM_rotate_key 20.0000 -20.0000
ANIM_rotate_key 0.0000 -0.0000
ANIM_rotate_key -20.0000 20.0000
Repeat for the same file in D:\SteamLibrary\steamapps\common\X-Plane 11\Aircraft\PMDG\DC-6\DC-6B\objects
Step 4: after loading X-plane and the aircraft, go to the topbar and select Plugins/Enable/Disabpe plugins.
Tick the box at PMDG DC-6.
Only this last step is required after every time Xplane is loaded.
Enjoy the DC6!
Credit for the rudder fix: