If you have a look at the manual (RTFM time! :laugh:)
http://www.destructor.de/firebird/gsec.htm[
^] you will see that "-add" is not a command line parameter - it is an instruction issued to it once it is running.
The first thing you need to do is log in on the command line, then when it is ready feed it your add command via the standard input (probably, I haven't tried it).