Here is a very similar topic:
The first Intel Macs are actually 32bit computers and those generation cannot boot from USB at all.
So the procedure is like this:
- burn a CD (!)
- start the boot manager
- Select the CD there
The rest of the procedure is otherwise the same.
The most important thing that almost everyone gets wrong: ... ... .... ***
FIRST OF ALL
The partitioning must be done by the Mac OS ("Disk Utlity") (!)
*** = (if ya aint need a Mac OS partition for dual boot you can skip this step and just choose 'auto' install options of the Linux installer - NOT RECOMMENDED FOR MacBook 2006 ! )