First make sure the version of outlook you use (32 bit or 64 bit) matches the version of zoiper you have installed. If they are not both 32 bit or both 64 bit it will not work.
You can find the outlook version here: https://support.office.com/en-us/article/What-version-of-Office-am-I-using-932788b8-a3ce-44bf-bb09-e334518b8b19#BKMK_Office2013Office2016
The zoiper version you can find in help -> about
Then enable the outlook contacts in zoiper -> settings -> preferences -> contacts
Make sure the profile name matches the one for outlook:
This link shows how to find the profile name: