You just need to supply a password. You can do it within the same command line with the following syntax:
openssl pkcs12 -export -in "path.p12" -out "newfile.pem" -passin pass:[password]
You will then be prompted for a password to encrypt the private key in your output file. Include the "nodes" option in the line above if you want to export the private key unencrypted (plaintext):
openssl pkcs12 -export -in "path.p12" -out "newfile.pem" -passin pass:[password] -nodes
More info: http://www.openssl.org/docs/apps/pkcs12.html