Well, I have finally managed to get the example to build. To do so:
- copy the TwoInts.h file from roscpp_tutorials to the include folder of your tutorial project.
- rename the namespace for the TwoInts.h file to your project name and go through and change every recurrance. i.e. since my project was under beginner_tutorials, change every roscpp_tutorials::TwoInt to beginner_tutorials::TwoInt
- in your add_two_ints_server.cpp and add_two_ints_client.cpp files, make sure the include refers to where you put the TwoInts.h (also, make sure its not 'AddTwoInts.h' like the tutorial)
- When you go to use TwoInts in your client and server file, make sure it refers to your namespace (for me it was beginner_tutorials::TwoInt).