Direct access to table cell properties

In tht project, I had to take over the InDesign source documents for some forms. Note that I do not have much experience with InDesign, so this may be very elementary.

Now, I have several problems with the tables:

• in one case, I have to add another row (which worked); however, despite all the properties I checked, the bottom border line does not show up. 

• Some table cells do have a background color, which I have to change, but I can't find out how to access intividual cells (Rows and Columns do have properties, but how do I display (and modify) for example the properties of the cell formed by the third column and the fifth row?

Thank you very much in advance for any hint.