Web services are not invoked from a web browser. You need to create a soap message and post it to http://localhost:5555/soap/rpc to invoke it. You can write a simple Flow service to do this using pub.client:soapRPC.
Also, don’t use soapRPC for anything beyond experimentation due to interoperability challenges discussed elsewhere in the web services forum.
Read the SOAP Programmer’s Guide and Web Services Developer’s Guide before more testing, if you have not already done so.