If you are trying to use v1.1 ('released' here) then you will have discovered the "missing resetarm.dat file" bug that Skimitar talks about here.
The quick fix!
The simplest fix is to download the file from here and put it in the same directory as the arm_1_1.py file.
Why does it need this file?
One of the irritations about using the arm is that after many commands you often want it to go "back to the start" - either because you've gone off in the wrong direction or because you've completed the task you were trying to do. There is no "reset" command built into the arm, indeed the arm is totally unaware of its current position, so to "undo" the movement we need to keep track of the current position. The program does this by holding each motor position in a python dictionary which it then "dumps" to a file (so that it remains 'persistent' beyond this session of the program). Next time the program is run it loads the position as its starting position. When you then want to "reset" the arm you simply run it with the command line switch -o and it reverses total movement accumulated since its last reset. This seems to work quite well - although there is backlash in the motors and if you "overrun" any of the motors it will get confused - as a result sometimes you will need to manually adjust it and then "zero" the reset position using the -z command line option. I'm going to explain how each bit of the software was intended to work in a series of blog posts tonight and tomorrow - so watch this space for more info.
The long term fix
The next release will include exception trapping for this missing file which will enable the program to skip the read instruction if the file is missing.