You're trying to add a waypoint to the map which you cannot do. In the upper section of the left window create a file list under My Collection. (Highlight My Collection and from the drop down menu select File/New/List and give it an appropriate name.) Now highlight the new list and search for the address, create a waypoint and you should be all set.
By the way I recommend that you install the map to your computer. You can do this using Garmin Express. With the maps installed on you PC you will not need to connect your GPS to use the map and it will redraw faster.
BaseCamp is a very good program but it can have a steep learning curve for some folks. Once you have a handle on My collection works the rest is relatively straightforward.
The tutorial linked in my signature is very helpful. It is designed for motorcyclists but the basics are the same and it will give you a good basic understanding of the various aspects of BaseCamp.