i downloaded 2142 from this site, then i went to run it, after install, and it said: d3dx9_29.dll was not found, and i couldn't play, can anyone help?
Yes, we can help and search can help Use this Download, install, should work.
thanks, +1, but i hate the demo, bf2142 sucks, so im going to still play bf2
So why ask for help to fix it?

How ironic.
Just because you don't like the demo, doesn't mean the game isn't/won't be good. You should still try out the retail copy, where you'll be able to get the unlocks, etc
See sticky at the top of this section.