You can unlock levels by selecting them in the menu and pressing the Page Up key. You will also have access to the console by pressing the " " key or TAB. From the console you can access some cheats:
Code- Result:
GOD - God Mode
GODSQUAD - God Mode for Squad
TIA - Unlimited Ammo
KT - Kill Spawned Terrorists
fly - Fly Mode
ghost - No Clipping
teleport - Teleport
stat all - All Stats
stat none - No Stats
allweapons - All Weapons
allammo - Get Ammo
loaded - All Weapons/Ammo
playersonly - Freeze Bots