Item: An individual unit (a Session or a Product)
Click Setup
Click Inventory
Click Add Item
If adding a Session, do the following:
Enter the Name of the session
Tick Service
Select the Category
Tick Unlimited
Enter Expiry Date (applicable from the date of purchase)
Update Members Rates: Select whether you want it to apply to New Members, All Members, etc.
Click OK
If adding a Product, follow instructions above EXCEPT:
Leave Service unticked
Fill in the amount In Stock