In both production release version 1.4.1 and a recent daily build (Sept 10), when I try to use the exp.txt file you sent, I did not get a traceback, but the error message:

Data line 5 of file either not selector/value or not tab-delimited

The original file in the documentation worked OK.  I text-edited exp.txt to see the problem.  In the line for residue 5, there are spaces in addition to the tabs.  The data line should just be:

(tab)atomspec(tab)value

but on that line there was another space between the atomspec and the value, and also spaces after the value.  After I removed those spaces, it worked fine, see attached file exp2.txt (it may be necessary to use Refresh....Menus in the Render tool if you had some failed attempts before that).  I put the file on my mac Desktop, so for me the commands to open a sample structure and assign the attribute were

open 121p

defattr ~/Desktop/exp2.txt

I hope this helps,

Elaine