I've looked at this website:
http://www.opm.gov/asd/hod/pdf/C060.pdf
which doesn't show anywhere I can see that a disability from illness or injury must occur in the workplace, which I thought at first was your question.
However, like most industrial insurance, any benefits will be dependent on the extent of your disability, whether you are truly covered by FERS, and whether you agency can find any possible place for you that will allow you to work within the limitations of your disability.
Have you expressed a need to your agency? That would be your first step. Well, actually, reading that PDF website and the rest of the regulations here:
http://www.opm.gov/asd/htm/hod.htm
should be your first step. If you then believe you are, indeed, eligible due to your employment and limitations due to injury or disease, you should contact your agency personnel office. There are forms at the end of the first website that can be printed out and should be, since you will need to begin documenting your claim, including records of employment, physician's reports of injury, etc.
It might be that, if your agency does not agree with your assessment, you may need to engage an attorney. Good luck.