I've never done anything with configurables, but looking at the editor they don't appear to have drop down menus, which is probably why your script isn't working. Instead, you might try:
Code:
field[cfgAllow1].text = "component.BaseSpell & Helper.InSpellbk & Spellbook.cHelpWiz"
Note: To find what the name of the candidate expression field for custom abilities is on a configurable, simply fill in some text in the editor on the appropriate line then click the "Fields" button to see what it says.
Edit: actually, you wouldn't even need to do this in a script. Just take the text above between the quotes and fill it into the primary canidate expression box in the editor.