Sydney,
You can respond to the "continued employment" part by sending copies of your last few year's annual tax returns (that prove you earned a living) and, if you have available, any documentation of services that you provided to your clients as a self-employed individual (say, contracts, invoices, maybe even letters from your clients attesting to the work you have done etc).
You should also write a cover letter explaining that you are self-employed (as permitted under the EB1-A category) -- thus the "employment offer" is not applicable in your case.
Of course, you may wish to consult a qualified immigration lawyer to be sure that you are responding to the RFE in a satisfactory manner.
Good luck!