I am very new at this and I'm not entirely sure how to explain this stupid and I'm sure easially fixable problem... I have the cursor and origin set to the center. But when I add mesh like a flat plane, it is added facing flat to whatever my view is instead of adding at 0x 0y 0z. So to add a plane flat with the grid, I have to be in top view (7).
Right after adding your object, uncheck the [ ] align to view option in the toolbar. If you don't see the toolbar, hit t and it will pop up on the left side of the 3d view.
Edit -- Just to add Zac's comment: you also can access this menu by hitting F6 after adding your object.