Not too sure what you have bought. monowall is an open source firewall that is free and readily available from
http://www.m0n0.ch/wall I can only assume that you have purchased a box with it already installed. Monowall can run on a PC as well as other hardware. It is based on a stripped out version of FreeBSD. I don't think that you would be able to add other applications to monowall. The monowall box will not add any additional security to your wireless AP. The wireless AP has it's own security with MAC address filtering and encryption, either WEP or WPA Current recomendation is to implement WPA if you can. You could implement a captive portal or add some traffic filtering rules which would make it a bit more secure. I would suggest that you have a look at the web site get a better understanding of what you have bought.
mark.