When using an external crystal as HFXO, use two pins, HFXTAL_P and HFXTAL_N. Pin setting (explicitly using Pin XXX as NFXTAL_P or NFXTAL_N) is unnecessary, so the default (Pin mode = disable) is fine.
Below is a sample code. The concept is the same for LFXO.

/* Enable HFXO as high frequency clock, HFCLK */
CMU_ClockSelectSet(cmuClock_HF,cmuSelect_HFXO);
/* Turn off HFRCO if not required */
CMU_OscillatorEnable(cmuOsc_HFRCO, false, false);

Depending on the sample code provided by the manufacturer,
/* Enable HFXO */
CMU_OscillatorEnable(cmuOsc_HFXO, true, true);
is described in some cases, but since the same thing is executed in CMU_ClockSelectSet(), it is practically unnecessary.

For settings when using an external oscillator, see another topic (How should I set the pins when using an external oscillator?).