Interfacing PS/2 Keyboard to 3.3V inputs
I doubt this is going to work. The output transistors on the 5V keyboard probably have a \$V_{CE}\$ that is too high to register as a logic low on the 3.3V FPGA pin. Since you didn't post any kind of datasheets, and probably don't know anything about the keyboard's output transistors anyway, trying this is probably the easiest way to see if it works. 

That said, since the outputs are open collector, the implementation is easy. 







simulate this circuit &ndash; Schematic created using CircuitLab

Pull them up to 3.3V with a 10k resistor to start. If you have problems with the communication, decrease the pull up value. R2 is a current limiting resistor to protect the pin. If it accidentally gets configured as an output, you'll be glad it's there. C1 is there to prevent over and undershoot, again, protecting the pin. It is optional, and if you choose to include it, you need to be very careful. On the transition from low to high, it must charge through the pullup and current limiting resistor. That rise time must be sufficiently short that it doesn't cause the communication protocol's timing to be violated. On the transition from high to low, when the transistor turns on, it discharges through R2. That fall time will be shorter, so there is no problem there.