Selecting the correct routing and socket card can be done by looking at the table in section Device Support table.
For Atmel Studio the correct routing and socket card can also be found by selecting the correct device in the STK600 programming dialog in Atmel Studio. A notification will display the correct routing and socket card to use unless the STK600 already has the correct cards mounted. More information on the programming dialog can be found in the Programming Dialog pages in the Atmel Studio help.
Note that some of the devices have a part-specific socket card (i.e., a routing card that has a socket). In this case, there is only one card to install onto the motherboard.