Hi
In order to enable ssel0, we need:
1. config ssel0 pin in pin config tool. for example (this screenshot is from LPC55s69, but same for your device)

after generate code, new pin_mux.c will include ssel0 pin initialization code
step 2. set TXSSEL0_N FIFO write data register to select ssel0 spi device.

Hope this will help you
Jun Zhang