You need to go to the Inventory dashboard and from there visit the On Hand Inventory and Alerts page. Once there, choose the locations you're interested in and click on the 'Show only products that need to be ordered' radio button. This will show just what needs to be ordered - based on the stock alert levels you must have defined earlier.
Pick the products which need to be ordered from the checklist on the left and click on Pin. This will copy them to the clipboard (icon on the bottom left) and save them there until you paste them elsewhere in Megaventory.
You can then open a Purchase Order as usual and after you've filled in the supplier and click to add a product row you can open the clipboard icon and click Paste. This will bring into the current Purchase Order all the items you need to order and significantly simplify the process.
Note that only products which are to be ordered from a single supplier can be transferred to a PO that way.