reinstall using usb does not work

Hi,

I just installed xubuntu 14.04 on an old macbookair1,1. But i screwed something up trying to get the wireless to work. Now i like to reinstall. Problem is that
i cant seem to be able to do that. When i try the usb boot disk I installed ubuntu with in the first place it does not longer come up when i press Alt. Actually nothing comes
up when i press alt except to choose a wifi network.

I installed ubuntu using a usb boot disk (macbook does not have a drive)..then i overwrote all mac os x partitiions (using the installer)..and installtion went
perfectly. But now i cant reinstall. What to do?