Hi,
Follow the steps:
1) Administration -> Setup -> General -> Report and Layout Manager
2) Choose the Purchasing A/P ->Purchase Order -> Purchase Order (Item)
3) Import your report here.
Or
Open Purchase Report, in the Tool bar, you will find Layout Designer, open it form PO. Choose Manage Layout, and It will open the Report and Layout Manager, and you can import the crystal reports. You can also define any report as a default from Layout Designer.
Hope it will help you.
With Regards
Ocean