Sadly, its not supported and Numbers for iPad strips out all the conditional formatting. If you edit the spreadsheet and send it back to your Mac Numbers or Excel, you'll have lost all the conditional formatting.
As a side note, I've found the performance on conditional formatting even on my Mac to be terrible. I can't even type consistently into cells without it missing keystrokes when there are complex rules in place. Until they fix the performance, there is no way they'll get it working on an iPad.
I really wish they would just ignore the conditional formatting on the iPad, but at least not strip it out altogether.
Conditional formatting does work in Numbers for both iPad and iPhone, but it involves using MS Excel. Either copy or create your spreadsheet into Excel and create your desired conditional formatting. Then transfer the spreadsheet to either iCloud Numbers or e-mail it to yourself and open it in Numbers on your mobile device. The formatting doesn't completely work in the iCloud Numbers Beta version yet (read further), but it will work in the latest version of Numbers on iPad and iPhone. Here's where it gets interesting: Even though iCloud Numbers Beta doesn't "show" the new cell formatting based on the data within, rest assured the conditional formatting is working in the background. To prove it, make your data updates in iCloud Numbers Beta then open the spreadsheet on your mobile device to see the new formatting. And, even more interesting: The snap shot of the spreadsheet in iCloud Numbers Beta will show the proper cell formatting! So there's definitely something happening on the Numbers Beta server that isn't ported to the iCloud App yet.
I'm using this method right now to track engineering drawing releases through our detailing group.
Please note that it is not just excel. You can apply conditional formatting on desktop numbers as well as other spreadsheet programs. It will carry over to the iOS version.
I think there is even one of the template files in iOS that has it turned on, therbuy shoing they designed the tempaltes on the desktop and transfered them to iOS.
Installed the latest version of IOS Numbers.
Further investigation shows that v2.1 of IOS Numbers can only handle a single-level conditional format imported from a desktop spreadsheet. IOS Numbers does not allow the user to create any type of cell conditional formating. When using conditional formating on text values, the current workaround is to create a single level conditional format in a desktop spreadsheet and import. Or, the user can use IOS Numbers' Checkmark or Pop-Up Menu cell format, neither of which allow the user to conditionally set the cell's fill or font color.