If the reason you are getting this error is because the DTD is unavailable, either because you're not on the network or the path to the DTD is different on your dev machine than in production, you can define a directory and place your DTDs there. Then, SlickEdit will look in that directory for the DTD if it can't find it in the usual place.
To set that, select Tools > Options > Network and Internet Options > URL Mappings. Enter the full path in "Default search directory" and click "Add".
This addresses the OP's issue in the linked thread that John posted.