runas tool

I want to use the runas tool to install printers when logged on as a regular user.

The windows explorer shell must be restarted with the local administrator’s security context. I restart explorer using ‘runas /user:machine name\administrator explorer.exe.’

The command prompt comes up asking me to enter the administrator’s password, but the text is grayed and I am unable to enter any text. This is the case for any machine I try.

The runas service is enabled.

please help

Thanks, Michael