I know that the directions on the new quests are really confusing. I admit, they were confusing to me too. I have a feeling that there is a typo in the quest dialogue (as there usually often is) that is causing the confusion. I have already contacted support about fixing it.
The TRUCK is west of the castle, not the castle west of the truck.
Here’s a map for y’all.
It’s on the Old Silverglade Road, just south of the Winery and almost directly west of the castle.
Hope that helps!
Edit: In the second day of the quests, when the cement is delivered, the truck driver does not finish his sentence in the quest dialogue. I have also contacted support about this. You need to go to the Winery basement to see the delivered cement.
Here is a video by Gun Dragonborn, showing the location of the truck.