Generally, we always strive to provide easy access to all DLCs in games that are available on our service. All you need to do is to enable additional content for the game you launched and it should be already available in the session. Be aware that sometimes a download procedure may be performed to install the additional content.
For example, you can always check what additional content is enabled in Steam via game properties.
In case you face any issues while trying to access specific additional content, please let our 24/7 support team know, and they'll be glad to resolve this issue as soon as possible.