You should have a CD of 'Trip & Waypoint Manager' supplied with your GPS. If you haven't, contact your supplier ...
T&WM is a very cut-down Mapsource, so I think Mapsource should install over it.
Another way, apparently, is to install
Garmin Training Centre first, then Mapsource. (Never done this myself)
A point to note, if you haven't got a software CD with your GPS - you DO need the Garmin 'USB driver' which is on that disk. The USB driver is obtainable free from the Garmin Website, as is 'Garmin Connect' which allows you to connect with BikeHike or BikeRouteToaster direct.
Install both those, to improve the usability of your GPS.
NB that Garmin Connect won't work with an Etrex Legend or Vista, unless you have already installed the USB driver. (This is often overlooked, because for most users who bought an Etrex new, the driver installed invisibly when they put the CD in ...)