Click on the Summary Function button (Sigma Sign)
Pick the field to summarize from the first pick list (Trans.AMT field)
The next pick list is the calcualtion type. it will change based on the field type you picked above it (e.g. you cannot SUM a text field).
Make it a SUM
in the SUmmary Location mkae it Group 4
Hit OK.
It will insert a Summary of that field at group footer 4 level.
Repeat the process but chang it the Summary Location to group 3.
Same again for group 2.
It will place a suammry on each of these group footers that will show you the SUM per grouping.