you dont unlock it at a level you unlock it by killing somone right before they get a 10 kill strick and you will earn the pot leaf emblem
The fourth level in Call of Duty Modern Warfare is No Russian (which can be skipped.)
Level 42
nothing
To get the cannabis leaf emblem you have to complete the denial challenge, which is killing someone who is 1 kill away from a 10 kill streak. This means you have to kill an enemy who is on a 9 killstreak, no more, no less. If you do this, you will get the emblem, I promise. You also have to be above level 60 on 1st prestige. (That's to get the High Command title not the emblem.) Go to www.callofduty.wikia.com
You unlock it at level 36.
Level 38
You need to level and complete challenges
The Lieutenant Rank is at Level 34
If you're talking about the museum level from Modern Warfare 2, then probably not. That was a one time thing.
There are 3 'weed title's' available in Call of Duty: Modern Warfare 2: Joint Ops: Get 15 assist kills (hurt an enemy player and have a teammate kill him) High Command: Become first prestige, level 45. Blunt Trauma: Get 2 melee kills while you have the Riot Shield equipped. Next to that, you also have a weed emblem: http://images1.wikia.nocookie.net/__cb20091126044537/callofduty/images/4/43/Emblem-weed.jpg To unlock this, you need to complete The Denier challenge.
you have to be level 29
Like in Call of Duty Modern Warfare 1 it is called 'End game'.