When you first start LSL2, you will be shown a photograph of a woman from Larry's "little black book", part of the original LSL2 documentation. Find the picture here, then enter the corresponding phone number to gain access to the game itself.
There is also an alternate means of passing this CP check. When prompted, enter the number 555-0724. This number (July 24th, Al Lowe's birthday) will be accepted, regardless of which picture is displayed. However...this trick only works on certain later versions of LSL2 (typically, the version(s) from the LSL collections)...it will not work on early release versions.
Additionally, using the special phone number will skip the game's introductory sequence entirely. If you wish to see it, you must pass the CP check by normal means.