I do not know that particular assembler, not compiler (please do not confuse the issue by calling an assembler a compiler - it is not).

some assemblers have a special way of declaring SFRs e.g. Keil:
sfr P0 = 0x80;

If you have a manual, look for declaring SFRs, if not the assembler probably have some files with it that describe e.g. 31, 32,51,52 look in those to see the method of defining SFRs.

The better way would be to download the free metalink assembler (available from metalink or philips) it comes with a full manual. The metalink assembler has become a de facto standard when not using the assembler that comes with whatever compiler you are using.

Have fun,

Erik