OK, I have found it using some example from stackoverflow.com:
NVIC->ISER[0] |= 1<<20 ; this compiles without error.
By the way: LPCOpen is a good platform, but the lack of documentation makes it very confusing to use.
What NXP calls documentation, it is worthless for a novice, and is not necessary for an expert. That's an automatically generated something, good for alibi, but useless.