To install anyway use the the option --nosignature
[code:1]rpm -ivh --nosignature <rpm file>[/code:1]
But its better to download the PGP signature from the RPM provider. Try searching near the place where you got the rpm fom a public signature. Its not always easy to find the singature key.
For example Freshrpms (has packages mostly for redhat) has it's PGP signature key at
http://freshrpms.net/packages/.
After you have the key you need to import it so that rpm can use it. It's something like [code:1]rpm --import <PGP Key file>[/code:1]
but I'm not sure about that. "man rpm" or Google can assist you there better than I.