To join 2800/3800 ME to 9800-CL WLC you will need console access. By default AP will not join. On the console go through ME setup, assign AP username and password and reboot. Once rebooted use console to login back into AP console mode.
(Cisco Controller) > apciscoshell
Log in to AP
(Cisco Controller) > en
Switch to CAPWAP mode:
(Cisco Controller) > ap-type capwap
AP will reboot and join WLC.