To load the package "tree" you do not write in a command but go to the "Packages" button in your R software and click that. Then scroll down to the button "Install package(s)..." and click that. It will give you a list of countries from where it will download packages for you. You may pick USA (PA 1). Now, you wait and it will come back with a list of Packages. Choose "tree" from the list and click that. It will save this package on your computer. Now again go to the button "Package" on your R software. This time click the first button in the list called "Load Package..." and choose "tree" from the list. To test if "tree" has been loaded type the command "tree". You will see the program behind the command "tree".