Support for the LPC546xx family is not built in to MCUXpresso IDE. But it can be added by installing an appropriate MCUXpresso SDK from http://mcuxpresso.nxp.com. For more details of how to do this, please see the supplied MCUXpresso IDE v10.0.2 User Guide.
For an overview, you might also like the watch the videos at:
and also see the blog article at: MCUXpresso IDE: Unified Eclipse IDE for NXPs ARM Cortex-M Microcontrollers | MCU on Eclipse
Regards,
MCUXpresso IDE Support