Basically its under Device Setup -> Add
However I urge you to read the Airwave user guide.
Page >124 covers adding devices manually in the 8.2.7 user guide.
You will need to provide SNMP credentials aswell as SSH/telnet credentials and target ip of the controller. Associated APs will follow.
Monitor read/only, is recommended.
Do note that you will need a sufficient amount of licenses in Airwave.