Ustream changed something in the last several weeks or so, not sure just what. It affects the ability to stream directly through the VidBlaster streamer. You can use the same Ustream codes and use FMLE separately as Amnon suggests. Frankly, I like have more control over FMLE settings, so I've not used the built-in Ustream streaming in VidBlaster in quite some time.

Using VidBlaster and then using FMLE to stream separately is covered in a tutorial I did a while back on Streaming to DaCast. Same principles, different codes.