Well, I'd say you don't. First, you'd have to be rooted. Then you could flash a newer android rom, but I'd bet that sense will *not* work on a Sony Ericsson. In fact, I'd say that if you tried to do that, you'd brick your device. So you could try to flash a sense rom, but don't!
And you don't really need sense do you? The launcher itself is a bit heavy (I'd stick with Go Launcher or ADW), and if you need widgets check out Beautiful Widgets or beautiful clocks or whatever... and Android Pro Widgets is pretty nice. And frrreeeeeeee!