First, you need to know if the driver for your wireless card is even installed as part of Ubuntu 9.04. Use lspci to find out what chipset your wireless card is. If it's any of the broadcoms then you need to search the forum for that chipset. I can almost guarantee that the same problem has been addressed here at least once. If it's not a broadcom, search anyway. If you can't find the answer here, on the Ubuntu forums or through Google, then post what the chipset is here.
When used properly, the search tool can be more effective than a new post.