Hey Respen,
The partner is spawned after the mission starts at any dead end road of the town, inside the farm or in the trees north of the town. There might've been a few bugs with him I think I've fixed since. Look for a zombie with an arrow.
Hope this helps, A huge update is coming very soon. I'm waiting to release at the same time as the PlayStation version. The frame rate has been increased so drastically with so much new content and polish.
Thanks for the support,
Rafi