signature fou4s

how come i get this failure with fou4s how can i force it to install New heimdal-lib: 0.3e-83 (old: 0.3e-27) [security, 288kB] Perform download/update (yes/no)? yes Getting sec1/heimdal-lib-0.3e-83.i386.rpm Signature verification failed for heimdal-lib-0.3e-83.i386.rpm (no updating)! heimdal-lib-0.3e-83.i386.rpm: MD5 sum OK: 67445feca5e272ffd302bd64e96511cc gpg: Signature made Sat Sep 21 00:11:47 2002 EDT using DSA key ID 9C800ACA gpg: Can't check signature: public key not found If you only got 'MD5 sum OK: ...' as result, then the package is not signed!