HyperLynx 8.1.1 allows R_series models only when they are between defined differential pins. Therefore, you need to have 4 elements in the model and design:
1. both pins in the [Pin] list
2. the two pins associated together in the [Diff Pin] section
3. the two pins linked in the [Series Pin Mapping] section with the series resistor model
4. the IBIS model assigned at the component level in order to apply the series resistor to both pins
So if you have pins 5 and 6 on the FPGA that are a differential receiver, your IBIS model should have these elements.
5 IN_P LVDS_25
6 IN_N LVDS_25
[Diff Pin] inv_pin vdiff tdelay_typ tdelay_min tdelay_max
5 6 0.1 0 NA NA
[Series Pin Mapping] pin_2 model_name
5 6 RSERIES_100
Then assign the component model to the FPGA reference designator using the REF file editor under the menu Models > Assign Model/Value by Reference Designator.
My syntax is slighlty different , but it worked.