Recently I have been working a lot over CRM 2011 and often get into CRM 2011 mindset of things. I was recently asked to have a button enabled only when subgrid record is selected. I went on to my CRM 2015 online environment and was trying to find the subgrid ribbon buttons. To my surprise, I could not locate them.
So I went a bit here and there. The user experience in CRM 2013/2015 for subgrids was changed . So first you need to click the button highlighted in green:
There is where you will see your subgrid ribbon and detailed subgrid:
I see people want to keep using main form ribbon to add buttons for sub grids and do unsupported methods to see what is in subgrids. I do not think it is the right approach. Anyhow on main form subgrid you can only select one row at a time.
We should go by new user experience for the new version. Still, If there is the same way needed, a better way is to develop HTML webresources.
Hope it helps!