Attached is a picture of my library entry for a similar package (Power-SO8).
Basically, you just need to use the "Connect"* command to assign the same pin on your MOSFET symbol (e.g. - D or S) to multiple pads on your package. Typically these style packages use three pins for the Source, four pins + exposed pad for the Drain, and one pin for the Gate.
* - more specifically, use "Connect" to assign the pin (ie - symbol) to the first of several pads (ie - package), then use "Append" to assign additional pads to the same pin as necessary.