The pump you received should have three adapters. One adapter is installed. Just pull it out and the outlet will have a hole. Place the largest adapter which is approx 1/2 outer diameter and install your 1/2 inch inner diameter tubing over this adapter.
If your tubing is installed on the inside of the tubing adapter this may be a problem but we can usually make an adapter for it. I would need a small piece of the tubing that connects the fountain pump to the fountain.
If none of this works for you please call our offices between 9-4 M-F Eastern Time and we'll try and help you that way. If you can take some pictures that would also help.