We use Thawte SPKI for our commercial certs. When I downloaded the cert I chose "Standard Certificate Format".
The other key part is that you have to add the Root and intermediate certs. To do that, you have to download the root certificates to add to the certificate chain. You can download them from Thawte's Root Download page
That will give you a zip file that contains all of their root certificates. You should choose "Thawte Primary Root CA/Thawte_Primary_Root_CA.txt" for the root CA.
Then you will have to choose the intermediate cert that matches your cert. In our case this was "Thawte Server Roots/ThawtePremiumServerCA_b64.txt.
If you don't know what your intermediate cert is, you can save your cert to a file and find out (assuming you have a Linux box w/ openssl installed) by doing:
and look for the CN in the issuer, then find the appropriate intermediate cert.
openssl x509 -text -noout -in cert.file