The hard disk drivers used on Apple computers with IDE drives are not currently compatible with this feature of At Ease. As indicated in the At Ease Read Me file, the dimmed security option is the correct behavior for At Ease when it detects a hard disk with an incompatible driver or when the hard disk has been partitioned with A/UX.
There is no workarounds for this problem. A new hard disk driver needs to be installed on the drive. Apple's formatting utility for IDE drives currently does not install a compatible driver.