For issues related to certificates on the Fortigate click here
For FortiAP troubleshooting click here
For FortiAP advanced configuration click here
If you are setting up your wireless network to authenticate users against active directory you can authenticate by user or by machine. In the case of machine authentication any PC which is a member of the domain will authenticate to the network via wireless on bootup. This prevents users from accessing the corporate network on unauthorized devices. Instructions can be found here.