Where is the P3 pin on the msp430?
The tutorial talks about a different model of the family which may have more I/O ports; however the 2553 does have port P3.

P3 is not a pin, but an 8 bit port therefore (up to) 8 pins P3.0 to P3.7. 

Whether the pins are actually available will depend on the package option. 

The 20 pin package (such as on the Launchpad) doesn't have enough pins to bring them out to actual device pins (though the logic will be there on the chip; not very useful but still visible to software as an extra register). 

However according to the 2553 datasheet, the 28 pin package option does bring P3 out as 8 pins.