You have a matrix and you want to add a detail row. One of the reasons could be to display extra detail columns but this is not so obvious in a matrix that works like a pivot table on a budget.
For a Tablix, this is fairly straight forward, if for example, you have deleted your detail group. See here http://msdn.microsoft.com/en-us/library/cc627469.aspx.
However for a Matrix, lets be frank, there are a few suggested workarounds, which in the end, will fleece you of your precious time and may not produce the desired end result.
Suggest deleting your matrix, and instead add a Tablix that already has a detail row, then add your row groups and column groups. Add your Row Groups, and to add column groups refer to my post here …