First of all, Welcome.
You may get timelier answers at
www.ab40k.org our authoring and bug chasing website.
You did not state how familiar you are with the software so I will start at the beginning... If you have already figured out some of this then skip ahead.
You have a copy of Army Builder?
Have you downloaded the Datafiles? When you open Army Builder you will a screen similar to this
https://imgur.com/jxY7C05
If you click the "Get Updates" button you will get this:
https://imgur.com/oJCNDWq
We are the 8th Edition files that are not in french.
Then after that gets updated you end up back at:
https://imgur.com/jxY7C05
Go ahead and click the 8th edition 40k files and "Load."
Then you get this screen. Click Chaos:
https://imgur.com/frCHBKg
Now is the part where everybody gets lost. Find the clipboard icon:
https://imgur.com/1qfqoqV
Its on the top near the middle. This is where we set the individual flavor of chaos.
You will get this screen:
https://imgur.com/tQlL1OZ
I reccommend turning on the "Campaign books" on because that is where the "Psychic Awakening" books are added on.
now scroll down further and Select "Heretic Astartes" because this is where we split the marines from the Daemons.
When you select "HA" in that field the different kinds of Marines pop up and you can select Deathguard. To add Poxwalkers to the list you need to click the Daemons box and Daemons of Nurgle too.
https://imgur.com/IxEu0TD
Now lets see if all the images posted correctly.
You should be ready to start list building.
P.S. I have downloaded and run a different skin.. All your windows will be blue unless you have re-skinned, do not be alarmed.