Please- why is 'voulu' changed in gender and number using 'avoir' ? I thought that only happened with 'être'.

If you look at the sentence in question (and, yes, it would be helpful to know what exactly you are referring to) you'll probably notice that there is a direct object coming somewhere before the participle. Something along these lones:

Les fleures sont très belles. Tu les as vues ?

If the direct object (les) precedes the participle, the participle matches the direct object in number and gender. 

Quite a bit late, but here is probably the sentence Jack was referring to. It also puzzled me, which is why I'm posting it now, in hopes of verifying the logic Chris used to explain its conjugation:

Q: How would you say: ''Those shoes, I wanted them immediately.'' ?

A: "Ces chaussures, je les ai voulues immédiatement."

Could a lesson be pointed to that could help to clarify this conjugation? My guess is it's posted at the B or C level, which is above where the test question was located (A2).