There are two basic ways to use Arduino microcontroller with Pachube:

• With an Arduino with an ethernet shield to connect to the internet: see Arduino + Ethernet shield.
• Using an Arduino tethered by USB to a computer to connect to the internet: see Arduino + Computer

If you have a Wishield (Wifi Shield) by async_labs, have a look at this tutorial.
http://community.pachube.com/arduino/wifishield