You read a previously saved boat file back into N2KAnalyzer by dropping down the file menu and right clicking on the "Open..." menu item. You can only read a boat file back into N2KAnalyzer if the current connection mode is offline or in other words, N2KAnalyzer is disconnected from an NMEA 2000 network (connection status is provided in the bottom right hand corner of the N2KAnalyzer window). If N2KAnalyzer is not offline (status window reads "Connected to NMEA 2000 Network") then you need to disconnect from the network by dropping down the file menu and right clicking on the "Disconnect from NMEA 2000 Network" menu item. Once N2KAnalyzer is "Offline" the Open... menu item found underneath the File menu can be used to read in an existing boat file.
Right clicking on the Open... menu item causes a file browser to open where you can locate a chose a file to be read into N2KAnalyzer.