Crystal frequency = the number printed on the crystal.
Oscillator frequency = crystal frequency, unless you are using an external oscillator.
PLL frequency = if the PLL is used, it is the same as the oscillator frequency multiplied/divided by a number.
Bus frequency = either the oscillator frequency / 2 (default), or the PLL frequency.
FCLKDIV is calculated based on both bus frequency and oscillator, as you can see in the algorithm described in the manual. You need to set it to a value between 150-200kHz. Also note that FCLKDIV is a write-once register, so you need to write everything at once, not bit-by-bit.
Can't give you a register value for 16MHz, but you will probably want to make a generic algorithm working with any frequency anyway.