I am sure there are some Explorist users out there that can and will correct me. I am not a Magellan user but you might try this.
Create a point of interest at your current location by pressing MARK.
You then edit the new position by using the Arrow joystick to highlight the field you want to change.
When you have the Location field highlighted press enter and see if you get a box that allows you to enter a new lat/long. Page 26 of your manual gives you a hint on personalizing a point of interest.