IIS uses pfx format cert. but the cert file downloaded from SSL For Free site is in .crt and .key type.
To converto .pfx file. Download openssl.
Run command to generate pfx:
openssl.exe pkcs12 -export -out "xgapps.co.nz.pfx" -inkey "private.key" -in "certificate.crt" -certfile ca_bundle.crt
Then:
1. Goto IE - cert store, delete current one if there is one
2. Open IIS, select "Server Certificates" to import it into "Web Hosting" store
3. On IIS, go to binding to select the newly updated Cert