From Edit-> Symbol Manager-> Click on the switch icon from the left, then click on the arrow to move the switch symbol to the right side
Then, on the upper right corener, you find name, give a name to your switch
Under the name, you find, Type, choose router 3600 (this in case if you are using NM-16 ESW module on routers 3640
as a switch
In GNS3, you can not add Switch IOS. You only can add Router IOS, but you can add a NM-16ESW slot to simulate
After that, Start your switc, then click on the add link icon, and choose "Manual" , then connect your switch regularly by using fastethernet ports
Hope this was clear.