If I'm understanding the question correctly, then you can't attach your email account to your wife's Zune Pass account. You'd actually have to get a Zune Pass for yourself. The only thing you can do at this point is use her Zune Pass account to download music that you can listen to on the computer or take with you on a device (if you had one).
What the 3 computers/3 devices thing means is that music downloaded from her Zune Pass can be downloaded on 3 separate computers and synced to 3 separate devices. But that all still attaches to one email/one zune pass account.
Does that make sense or did I totally misread the question? :-)
Marques Lyons (tromboneforhire)
Zune MVP
Contributor, http://www.insidethecircle.net