They are defined in the file msp430g2553.cmd
.
/************************************************************
* DIGITAL I/O Port1/2 Pull up / Pull down Resistors
************************************************************/
P1IN = 0x0020;
P1OUT = 0x0021;
...
PS: I'm using CCS. The file is located at path\to\ccs\ccs_base\msp430\include
along with the header file msp430g2553.h
.