after checking "has Error Bars" you have to define the error range for this (main-)expression. To do so expand the according expression using the [+] in front of it. With the last option you can define the range of the "Error Symmetric". For ex. "Expression * .1" to show an error range of +/-10% of Expression ( normally identical with the choosed Main-Expression).
You can just tick the box, then expand the "expression" and in the bottom most selection just add in something like "100" to give an error bar of 100. Which stage are you having trouble with?
