Hi,
my experience is that you can boot Trisquel from a USB easily if you have
already a dual boot with grub e.g. with native macos and ubuntu.
First, connect your trisquel usb while the computer is still off;
When you start boot and grub lets you choose between your systems, after you
choose linux at rEfit start screen,
(don't type the quotes at your prompt line)
press "c" to enter grub command line;
type "ls" to see which are your drives, and localize your usb disk as grub
sees it. E.g, your hard drives could be like (fd0,'number'), and (fd1) would
be your usb, typically listed in last position;
type "set root='(fd1)'" assuming (fd1) was your usb drive;
type "chainloader +1";
type "boot";
And voila, that should start your mac from the usb, letting you test trisquel
