You almost have it, the function is getFieldString, not getFieldValue. If you replace the function name in your example it should work as expected.
If you want some more in-depth information, here it is:
A tiddler is just a special type of javascript object. The object returned by this.wiki.getTiddler(tiddlerTitle) is in the from:
{fields: {
title: tiddlerTitle,
text: some text,
tags: tag
},
cache: {}
}
So to get the field values you use something like this:
var tiddler = this.wiki.getTiddler(tiddlerName)
var fieldValue = tiddler.fields.fieldname
where you replace fieldName with the actual name of the field. To fit with javascript syntax you may need to reference field names like this tiddler.fields['field-name'] when there is a - or a . in the field name.
The getFieldString function has some extra steps so an empty string is returned in the case of missing fields or invalid field values, and it uses the build-in stringifying functions for fields like the tags field.