Litmus is a paid feature hence it would be good to check if its included in your contract.
Then it actually depends on what flavor of campaign are you using ACC or ACS. If you are using ACC then what is the architecture type ( Fully hosted with Adoe or Marketing instance is hosted by you and mid server is hosted by Adobe )
In general to activate Litmus on Marketing instance following steps are required assuming you host the marketing instance and Adobe hosts the mid instance and once the below mentioned steps are complete raise a support ticket for rest of the task to be carried out on the deliverability instance
1 - Install the Inbox rendering package via Tools>Advanced>Import a package
3 - Setting up options Server>Administration>Platform>Options. Search for cuid ( There should a value mentioned there which matches the value on delievrability server, which can be checked with support team
4 - Edit serverConf.xml
It will also be necessary to edit serverConf.xml to allow a call to the Litmus Server. In serverConf.xml, add the following line to the <urlPermission> section: