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.
If you still do not see a change one the frontend of your website you'll need to reindex the site.
SYSTEM > INDEX MANAGEMENT
Checkmark "Category Products" and "Stock Status" and hit Submit.