The first steps with the SPLA program is obviously signing up on the microsoft website under the hosting parter scheme.
To actually utalise SPLA you will need to contact Microsoft directly or via a SPLA dealer such as the one you posted. You will need to fill in some paperwork and that will be sent to Microsoft before you are accepted into the program.
Once all of that is done and accepted you will only deal with your SPLA dealer. They will provide you with media (usually charged) and some dealers offer free downloads of the software aswell as your product codes that you qualify for.
You will only then pay the SPLA dealer and not Microsoft directly. All your contact typically will be with the dealer.
You can usually download the software from your SPLA Reseller and also get your licenses from him directly. You also pay them monthly and make sure you are not late or Microsoft will terminate your agreement