When you first join PriceYak, PriceYak will show you all of your existing listings for which it does not know where to source the item from. To see this list, go to Fix Problems -> Track Unknown/Untagged Listings.
You can individually enter information for the product source, or you can add them in bulk by uploading a CSV through the Bulk Tagger, which is the blue button on this page that says 'Bulk Tag Listings'
The format of the CSV file should be "itemid,source,product_id" (for Amazon-destination, use SKU instead of itemid). For example, one row might look like "222136387160,amazon,B00A850UVG" (without quotes). You can see a sample file here.
Is excel jumbling your numbers like "1.23123E+12"? Click here.
Warning: If you upload incorrect data, your listings might be tagged improperly creating mayhem. Please upload a small sample file first to make sure everything works is working correctly.