Why can't I find Dayspring Canyon?

This location can sometimes be cut off if you haven't finished the prerequisite quests in either Dawnguard or the Companions questline. Most notably, if you have started the companions questline first, sometimes the entrance will be inaccessable until you finish the quest with Vilkas(?).

I would recommend finishing a couple more companions quests before continuing with the Dawnguard DLC

You can find it next to "Broken Helm Hollow" it's to the bottom right of it. Try going to the area, there is a cave that will take you there. The place is "Stendarr's Beacon". If that doesn't work, try looking for an update.