Start ISP using the "map" command in the developer console.
map c8m1_apartment (to start L4D1)
map c1m1_hotel (to start L4D2)
This is only required the first time you start a game, once you properly started ISP you will be able to choose a campaign by pressing F10.
To uninstall ISP:
1 - start the mod and restore the crosshair with the "6" key, quit
2 - remove the .vpk from the addons folder
3 - start the game and reset keyboard to default from the game menu
4 - (optional) if you forgot step 1, type "sv_cheats 1" followed by "crosshair 1" in your console, then type "sv_cheats 0"