Don't click on that green Sign Up Now, that's for those that don't already have a WGT account.
Click on one of the other black buttons...
Play On Windows if you are using a Windows computer.
Download On The App Store if you are using a Mac.
Get It On Google Play if you are on a device running Android.
Don't know what the Amazon one is, maybe for a potato or something, lol.
Anyway, clicking one of the aforementioned buttons will download a file to your machine. Find and run that file to get the new version of WGT.
Once you are up and running, sign in with your current WGT login, there's no need to make a new account. All your equipments and records will be waiting for you to play with there.