The DL1000 has a wet clutch system: The clutch plates are "lubricated" (for lack of a better word) with engine oil. If they've been sitting for a while, that oil may have caked and deteriorated somewhat. It should not be too hard though to get them to separate, but it might require a bit of force.
Here's what I would do. First, see if you have enough hydraulic fluid in the reservoir and that the fluid is still good. (Fluid should be clear, not black, and officially needs to be replaced every two years.) Then, inspect the clutch for proper functioning: Open the oil filler port and look down onto the clutch plates. Engage/disengage the clutch lever and you should see the pressure plate engaging and disengaging. It won't be much but some movement should be visible.
If that checks out, then you need to break the clutch plates free. In neutral gear, warm the engine until you have two bars showing. Then shut the bike off. Disengage clutch (pull the lever), put the bike in first or second gear, and with the clutch disengaged (lever pulled), rock the bike forward and back until the clutch plates break free. If you look through the oil filler port carefully you should now see the drive plates and the driven plates move independently of each other.
Once the clutch plates are free you might want to ride the bike a while in stop/start traffic so the clutch is used a lot. That circulates the fresh oil over your clutch plates and helps get rid of any caked oil. And then you might consider doing another oil change, although if everything works fine, I would just forget about it.
What you've done so far (Engine off, 1st gear, let go of the clutch lever, and then push the bike so the engine goes through a revolution) does nothing for the clutch plates as they are fully engaged at all times.