How did you install Ubuntu on it? I had a real pig of a time getting an Acer netbook to boot to a live CD, I eventually booted it using USB. I've never tried booting from a flash card, but it may well be possible
if you're having trouble getting it to boot then the first step is to change the BIOS settings so that Hard Disk is below whatever other media you want to boot (in this case, it could be the CD, DVD, Flash or USB)