It's just about setting it up. Without more details, I have to guess what you've already set up. So I guess the iMac is set to your Apple ID. And then the wife's iPhone is set to her Apple ID.
On her iPhone, you want to check that she is logged in to iCloud in the Settings app. Also, make sure she hasn't turned off location services. See Settings, Privacy and make sure it's on. Then look further down and make sure it's enabled for Find My Friends.
Then she goes to the Find My Friends app. Press the add button. Then enter your Apple ID email address. Select Del indefinite.
It should be. Now, if you use Find My Friends from iCloud.com or use the iMac Notification Center, see the location of your wife's iPhone.
See Configure and Use Find My Friends for more information.