Package contracts can be attached to a package on an account as needed or included by default on packages by configuring the contract on a package's package frequency in the product catalog.
Adding contracts to account-packages
See the Account Packages help for instructions on adding a package level contract. The Account Packages help can be found by loading an account and clicking on the left sidebar options 'Account' > 'Profile' > Packages tab > help icon.
Adding contracts to package frequencies
Load the packages screen and select a package frequency sub-element under a package. From there you can set or change the contract on the package frequency.