Like LPCX Support suggests, download MCUXpresso IDE and also create an LPC54608 SDK on https://mcuxpresso.nxp.com. The SDK for LPCXpresso54608 is available as a direct download.
Just drag & drop the SDK_.zip file in the "installed SDKs" pane to install it.
From the quickstart panel you can now import the SDK examples if you like. When creating a new project, select your processor in the same way as with LPCXpresso (project properties, C/C++ Build -> MCU settings). The 54608 can be found in the "SDK MCUs"section.
Rob