as far as i know you can register from any country, but if you have any problems opening the site, which i did, you can just use a VPN.
I wouldn't recommend doing that! At least, contact customer service and ask them if it is OK to use VPN to access the casino or wait for their representative to give you the right answer. If the use of VPNs is prohibited then you mace troubles later when you want to withdraw your money.
I checked their terms of service and didn't find Austria on the list of restricted countries so not sure why it doesn't show on the drop-down list?