Tasker and Locale Execute Plugin root command not working in 2.3.5
The plugin you're using is deprecated; Tasker now has built-in support for shell commands (under Misc->Run Shell). You can use root with this also.

EDIT: It's too bad that didn't work for you :( The last option I can think of is using Secure Settings Plugin. This can run shell commands too, and has the option of returning STDOUT and STDERR to Tasker as well. Good luck!

Also make sure that in your Superuser app you've set it to ask for permissions (or even automatically grant it - turn that on, run Tasker for it to get su permissions, then set it back to "ask").