Connecting Ethernet Shield + Arduino

Hi everybody:

I just connected today the Ethernet Shield to the Arduino and tried to make it work on internet for example posting something to twitter, but it's not working.
I tried several sketches of different codes to post or to retrieve data to/from twitter, from the botanicalls twitter to some sketches in this forum, but there's no way to make it work -it always says either timeout or impossible to connect, and that makes me wonder if it really is a problem of IP assignment or XPor Direct configuration.

I followed the instructions in Ethernet shield but there's absolutely no way.
I do not know if the problem is about assigning some IP to the Arduino -as I said, I configured it as they say in ladyada-

Thanks a lot for your answer!

I can ping the Arduino from the same network, but still I can't post to twitter.
Is there any other way to know if it is online and if I can do something with it?