how to assign the pin for DDR sdram?

In pin planner, in the pin list at the bottom, there is a column called I/O standard. You can set it there.

You should refer to the pinout of specific device at http://www.altera.com/literature/lit-dp.jsp and look for DQ/DQS columns for proper pins. This can be done in Quartus II pinplanner too by right-clicking the pinplanner background and select "show DQ/DQS pins".