The RX Sampling slider that allows you to adjust the PMA_CDR_SCAN is supported only when the RX_DIVSEL_OUT_* is 1. If this is not the case, the range of the valid values that allow a correct received signal are reduced and the slider cannot be used.
To work around this issue, check the R_DIVSEL_OUT current value, open either the Edit Line Rate dialog or Edit DRP dialog. If RX_DIVSEL_OUT_X is 1, you can use the slider. If RX_DIVSEL_OUT is not 1, you should set the slider to a value of 64. This will be fixed in ChipScope 9.2.