As you downloaded the portable version that includes both 32 bit and 64 bit versions, did you succeed to launch the 64 bit version it contains?

While I'm at it, can anyone explain the Java web start vs a normal program install?

It's a way to run Sweet Home 3D application on a PC where Java is installed but the user don't have the right to install more software. As it's easier for developers and users to manage program updates, it's also the preferred way for me to distribute beta versions for the moment. The main drawbacks of Java Web Start are that it obliges the user to install Java first and some plug-ins don't work with it.