Fmin is the lower frequency range of the PLL design you specified for ADIsimPLL and ADIsimPLL restricts operation to this range. In this case you have specified a fixed-frequency PLL, so Fmin = Fmax and ADIsimPLL won't let you change the frequency.
I see that the helpfile link has broken to this dialog - we will fix that in the upcoming release and make sure that the help page includes details on Fmin and Fmax.