You need to drive the trailer into the loading zone. Then you choose Load from the menu at the top. After that you need to press start.

Lucky you, I can't even get the new mission with the cattle to do anything, I click start and the window just disappears momentarily then reappears without the mission starting no matter how many times I click start.

My mission loads ,but the dealer has no animals for sale.I own 5 bulls when it starts. The mission will not progress as I cannot buy animals. There is a route marked on the map but it just goes in a big circle. Do you have to go somewhere to buy the bulls or is the dealer menu under animals supposed to show some for sale?

Are you playing on a old savegame file @MrSpecies7

No , new game and even a reinstall because of patch. It also crashed escaping from the dealer window after 2 or 3 attempts at seeing if he had any stock.So there may have been something else going on.I did send the crash report of course.
So I take it this means the dealer should have bulls for sale in the animal/dealer menu?

Yes he should. You can try deleting the Cattle and Crops folder in your mygames directory just to make sure.