There are several ways to load autolisp routines into in AutoCAD.

Option 1:
The most common is to add a (load "x") into "acad.lsp" (where x is your lisp file name - parenthesis required). Be sure your acad.lsp is in the search path by typing (findfile "acad.lsp"). If you get a path (and the path is correct), it sees it and will load it at the start of AutoCAD. If you do not:
options -> file tab -> search path -> click browse -> add the path with the location of acad.lsp

Option 2:
a. If you desire to run the lisp only one time, you can (load "x") at the command line.
b. You can also paste the lisp function on the command line.

Option 3:
A far easier way is to use Appload.

Once loaded, look for the characters that come after C: and before (. That is where the name of the command will be. You run it by typing that. (btw: You can change the name of the command to anything you want. Some characters are interpreted by the lisp compiler to be commands, so use only letters and numbers in the name if you want them to load correctly.)

If you use Autocad LT, you purchased a product that deliberately keeps lisp out. Try this site.