Net demo web client is great for testing.
I don't think you'll be able to make it work with the PFX, though.
A PFX contains both the certificate (with its public key) and your secret private key. And it's password-protected.
To split apart a PFX for use with NetTalk, use openssl (it's installed automatically with any of your netweb server projects).
When you use openssl, the first private key file you'll get will be password-protected.
You can also run one more openssl command to produce an unencrypted private key file.
Openssl pkcs12 -in MyDomain.pfx -nocerts -out MyDomainEncrypted.key
Openssl rsa -in MyDomainEncrypted.key -out MyUnencryptedDomain.key
Openssl pkcs12 -in MyDomain.pfx -clcerts -nokeys -out MyDomain.crt