I am looking for a way to export the information in the DHCP monitor to a file that I can manipulate...preferably Excel. How can I do this?
That depends on your skill level and determination.
1- the easy way: pull the info via SNMP (v3). A python script could interface with SNMP and write an XL file.
2- use the REST API to directly access the FGT internals. Pull the data, convert, write an XL (or CSV) file. This will call for some effort, skills, and probably membership in the FTNT Dev community.
Another way if you have a preference with bash/SSH : a server would pull the information by doing a "execute dhcp lease-list"