Mosfet Power Dissipation
$$P=IV$$

This is true for any circuit branch. For a MOSFET, if you aren't switching it extremely rapidly, \$I\$ is the current through the MOSFET channel. \$V\$ is the difference between the source and drain voltages. 

If you are using the FET fully switched, this will be approximately \$I^2R\$, where \$R\$ is given by the \$r_{ds(\mathrm{on})}\$ parameter from the datasheet. And "fully switched" means with the gate voltage in the range given in the specified conditions for \$r_{ds(\mathrm{on})}\$