Ivan, after you installed Ultimate Traffic did you run it and select Compile? I did that now that I have the update installed and followed the prompts. Everything runs fine and I'm hearing ATC traffic with Alaska, United, and the other airlines I would expect around KSEA.
Edit: I loaded up Fs2004 at Logan Intl (KBOS). Hopefully this is the same Logan you mentioned. Everything was fine there. I had 2 planes ahead of me and 6 behind when I finally got to the runway and took off. SO I think if you haven't done it yet you should load Ultimate Traffic and select Compile. I took a screenshot and circled Compile for visual reference.
Just follow the prompts from there and everything should run fine. If not let me know and I'll see waht else I can do to help.