Hi Pete
Before i got my lowrance gear i used a garmin GPS 72.
To manage all my marks i used a free programme called Easy GPS and i had all my marks sorted into different files depending on the area i was heading to ie Goldy,Sunny coast,The Bay. All i had to do is plug my garmin into the computer and download the ones i wanted.
I still use the programme as a backup for my lowrance plotter. In the settings on easy gps you can set it up for your unit and you can convert the file types on there from Garmin to lowrance or whatever other units they have on there.
I'm no computer whiz either so if i can do it can't be too hard.
http://www.easygps.com/
Cheers Dazza