Tableau is a 32 bit application and you need to connect to database with a 32 bit driver, even if your OS is 64 bit (xp 64 bit or Windows 7) - please check this first.
I found this thread searching for something similar and i would like to point you to this link on IBM site
ODBC diagnostic and performance tools
Also, for your IT staff please let them read this threads
I hope this helps.
I posted this on a similar thread, but since this one comes back higher in Google search results I'll post it here too. I hope that it helps.
I started a Tableau trial last week and was having a lot trouble getting Tableau 8 and our iSeries to play nice together via ODBC. What I finally ended up doing was to create a Linked Server on a MS SQL Server that I can access. So, I'm essentially bouncing a Tableau Custom SQL query off of a SQL Server to the iSeries. It's probably not the most efficient solution, but it provides me access to the data without creating a temporary data store and Tableau stopped complaining.
Link to an article explaining how to create a Linked Server:
I have only created one dashboard with the iSeries data so far, so I don't have a lot of comparisons on performance. All of my dashboards from other data sources are much quicker, but they have a fraction of the data. My iSeries dashboard has three joins and is pulling over 600,000 rows and takes about 15 seconds to load.