how do you sent a tweet from arduino via arduino wifi shield?
The code you link to only looks at a user's tweets.

Change 'arduino' in the following line to the twitter account you wish to read

client.println("GET /1/statuses/user_timeline.xml?screen_name=arduino HTTP/1.1");


To send posts you need to send a HTTP POST packet, e.g. see 

https://dev.twitter.com/docs/api/1.1/post/statuses/update

For the other api commands: https://dev.twitter.com/docs/api/1.1

But this all requires authentication:

https://dev.twitter.com/docs/auth/oauth

The people at stackoverflow will know how to do it!