It can be done, but it's a bit of a pain. You need to use a VPN or DNS service that makes it look like you're based in the US. You also need to provide a US address and a credit card number that's tied to either a US bank or a Visa or Mastercard gift card that's registered in the US.
It's probably not worth it at this point, though, as TCM and Criterion are launching a new streaming service called Filmstruck this fall. It's possible that the Warner Archive will fold into that service - some have already noticed that the Warner Archive's selection has become quite poor in recent months.