Try prefixing the certificate filename with “./”, or using the full path. From the curl manpage:
If curl is built against the NSS SSL library then this option [–cert]
can tell curl the nickname of the certificate to use within the
NSS database defined by the environment variable SSL_DIR (or by
default /etc/pki/nssdb). If the NSS PEM PKCS#11 module (lib-
nsspem.so) is available then PEM files may be loaded. If you
want to use a file from the current directory, please precede it
with “./” prefix, in order to avoid confusion with a nickname.
(emphasis mine)
Alternatively, some instructions for importing into the NSS certificate database are here, although I have not used them:
http://rcritten.fedorapeople.org/nss_compat_ossl.html