You don't need to become your wife's representative in order to do her taxes. The CRA understands that spouses and family members will do tax returns for each other. You would only need to be her representative if you needed to call the CRA on her behalf.
If you do wish to become your wife's representative, you can contact the CRA if you need assistance with the process.
If you are in Canada or the United States, you can call the CRA at 1-800-959-8281. If you are elsewhere, you can call 613-940-8495. This page has more ways to contact the CRA: Contact the Canada Revenue Agency