The optional rules listed in The Force Unleashed Campaign Guide regarding the Dark Side is in effect. In essence, a Force Point may be spent to gain temporary use of any Force Power with the dark side descriptor. It can then be used once during that encounter. Likewise a Force Point may be spend to gain access to a dark side talent. This lets you use the talent (or talents if you spend Force Points to buy more than one) for the entire encounter. However, your character gains dark side points equal to the number of Force Points spent in this manner.
Refer to the TFUCG for complete rules.