According to the schematics (and I can see it on the photos), there is an 8MHz crystal feeding the external oscillator pins.
Have you tried configuring the clock control register(s) to enable the external oscillator and use it to drive the PLL for the system clock? According to the F401 reference manual, the device automatically uses the internal 16MHz internal clock (HSI) on reset. It looks like you have to manually configure the use of the external oscillator (HSE) through the main PLL in order to achieve higher system clock speeds (e.g., the max of 84MHz). See section 6.2 and 6.3 of the F401 reference manual for details.
This board is currently in the mail on its way to me, so I can't actually verify this yet. Once I get it, I'll try to do the clock configuration myself and let you know how it goes.
After playing for some days with the boards - a warning : The implementation of the mbed library needs a lot of improvement to use this boards ! The processor is running at 16 MHz and I'm wondering about the slow SPI speed... Most of the Features from the list above are not implemented today. If ST want to advertise with "mbed enabled" they have to spend more time. Also they did not spend the 10 cent for a crystal to have a stable timing source for the PLL...
Without a solid debugger - wait for a better implementation or you will wast your time.
Peter, (a experienced mbed user )