The Portfolio Object itself cannot have a Department OBS unit. However, the 'Portfolio Investment' Object can have the OBS Units configured.
There was a defect where this configuration was not working, but was fixed in v14.2 as per CLRT-75920
(KB TEC1439639 is in the process of being updated with the resolution status that it is fixed in v14.2)
When you register the Department OBS Unit , pick the one that cooresponds to the investment type , like 'Department OBS (Project)'
And then add it into the Investment portlet layout and you should see the Department OBS for each investment in the portfolio
If you need help to understand how to 'register' attributes you can check out this YOUTUBE video
What is Attribute Registry in CA Clarity PPM v13.2? - YouTube
Also, the current functionality with respect to the Portfolio child relationships is restricted at this time.
Check out this article:
http://www.ca.com/us/support/ca-support-online/product-content/knowledgebase-articles/tec615513.aspx