How do you write a letter of compliment, a letter that will get an employee noticed? What info should be included? Should it be typed or handwritten? Do letters of compliment really help an employee?

We had a wonderful experience with our concierge and feel he should be recognized.

I'm not sure how important form is. Just make sure the message gets to the right person. In the case of a hotel, that is the general manager. For a concierge, a tip might be more valuable than a complimentary letter.

I actually wrote a letter of compliment just a day ago to a hotel I stayed in where I had a fantastic stay. Waiting to see if I get a reply.
It was more of a general letter (actually an e-mail) and I addressed it to the management of the hotel.
I don't think it has to be in a special format, just make sure you write about who you are, when you stayed there (in case he/they might remember you), and of course about your nice experiences with a few positive details.

If you want to compliment the concierge I suggest you address it to the front desk manager since he or she would most likely be the boss of the concierge.

Do you have the name of the concierge?
If you do, mention it clearly! Many hotel chains offer bonuses to employees whose names are specifically mentioned by customers e.g. in letter of compliments, the "opinion cards" given at check-out etc.
So he might get a nice surprise at the end of the month...