The DK-6 doesn't screw in: it fits underneath the eyepiece. You unscrew the eyepiece, fit the DK-6 and screw back the eyepiece.
A tip if you don't find a replacement: look for a used F801 or F90 cheaply. The eyepiece is the same. BTW, The F4 eyepiece fits on the F100, but it is an overkill for the F100 as it is also the opposite way!