In your bin directory you will find a kernel and an ext2 root file system
when it's finished. Just run it like this:-
-bin/openwrt-uml-vmlinux ubd0=bin/openwrt-uml-ext2.img con=null ssl=null ssl0=fd:0,fd:1 con0=null,fd:2 init=/etc/preinit
+bin/openwrt-uml-vmlinux ubd0=bin/openwrt-uml-ext2.img con=null ssl=null ssl0=fd:0,fd:1 con0=null,fd:2
The uml will start, and eventually the serial console of the uml will be at your
console prompt. If you would like it in xterms, substitute con=xterm and con0=xterm.