I have been using my Magellan Explorist 200 since I started a few years ago, and have been very happy with it.
To enter a point of interest, press the lower left most button labelled mark. It has a little thumbtack icon on it, unless yours has worn off from use like mine
I suggest being outside where you can get a signal lock. The Explorist I have will prompt me to enter setup screen if it cant find signals within a few minutes.
If the GPS hasn't got a strong signal lock yet, you will get a warning that there is no GPS fix. Press straight down on the joystick to accept the yes response (no lock is fine, your going to input the co-ordinates from the cache page anyway)
The next screen will have a field for an Icon, Name, Location, Elevation, Message and a Save or Cancel button. Use the joystick to move the highlighter to the field you want to edit.
Once the field is highlighted, press straight in on the joystick to toggle on edit mode. At that point the joystick starts acting like a scroll wheel. Once you have the field edited the way you want, press in on the joystick again to toggle off editing.
May take a bit of playing, but once you have your point of interest entered and saved, then you can use the GOTO function. Press the lower right most button GOTO, with the hiker man icon on it. Use the joystick to select My points of interest. Scroll to the one you want and select it. Now on one of the Nav screens you will see a distance and bearing to the point of interest you entered.
Hope this helps.