Resonator characteristics for USB applications

From slau208 MSP430x5xx and MSP430x6xx Family User's Guide

For crystal operation, a 4 MHz or higher crystal is required. For crystal bypass mode of operation, 1.5 MHz is the lowest external clock input possible for CLKSEL.

Basically, you can use any XT2 CLK that will be inside USB spec tolerance +/- 0.25% (2500 ppm) for full-speed bus. If you want to use USB BSL than XT2 CLK must be 4, 8, 12 or 24 MHz. Here is my USB PLL configurator...

http://forum.43oh.com/topic/3904-usb-pll-configurator