Since none of the examples I found worked for me, I put together my own example which shows how to use the MSP430G2553 as an I2C slave. The MSP430 takes some simple commands. One for switching the build in LED (P1.0) on/off, one for reading the state of the build in button (P1.3). Also a master example is provided. This example I run on a modified TP-Link MR3020 router which runs OpenWrt. It is also possible to use a Bus Pirate as master (see end of this document).
For a quick start to setup the I2C slave, the TI USCI I2C slave code from slaa383 was used and slightly modified to work on the MSP430G2553 and the msp-gcc.
The sources could be found on github.