I've just purchased the Symbiosis DLC on Steam and it is not installing. It remains greyed out in my Steam library and when I launch the game it states that Symbiosis is deactivated. I've tried rebooting my pc but that made no difference. Please help to get it installed. Thanks in advance.

There are a couple of things that MAY work. I found these here: https://steamcommunity.com/discussions/forum/1/540744935255429288/ 

Basically, what you may want to try is unchecking and rechecking the dlc enabled in the steam dlc sub menu. If that doesn't work, I would then "Right-click the game in your Steam library > Properies > Local Files > Verify integrity of game cache " (as from the linked post).