You may be confusing the terminology, which is easily understandable in this case. The option you are selecting in the RSA II web GUI is "single-user" in terms of the number of remote console sessions that will be allowed to operate simultaneously; it has nothing to do with the mechanism to boot the OS into single user mode.
To boot linux into single user, the steps can be easily found with an Internet search. A quick summary would be to interrupt the boot loader countdown to modify the boot command, in the case of GRUB with the character "a" to "add" to the predefined boot arguments. Then you would add the "single" boot command argument and resume the boot process.