If you go to the advanced search options in Trulia you will see a tab for MLS ID. The problem is you need to know what city your looking in. If your looking for a home in the Northwest Multiple Listing Service (aka Greater Seattle Tacoma area) you can search by MLS Number on my site http://www.theeastsiderealestateguy.com/search/
Right hand corner you will see Listing #:
Put the MLS listing number in there and if the property is available it will show up
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If you are on the search window in the lower right hand corner of the green box is a menu for "more search options." On that window is a tab for the MLS ID.
Call me if you'd like any more help. I think you have my info from FIABCI.