To add inventory to a product:
CATALOG > MANAGE PRODUCTS
Then choose the product you would like to add inventory to.
On the left side of the product is the product menu, click the Inventory tab. From here you will update the quantity of your product.
Once you are done updating the quantity, click Save.
Now you will need to flush cache and reindex.
SYSTEM > CACHE MANAGEMENT. Select Flush Cache Storage at the top right of the page.
SYSTEM > INDEX MANAGEMENT. Select "Category Products" and "Stock Status" and hit Submit next to Actions: Reindex Data.