Scrub as much of the plaster loose as you can with a stiff scrub brush. Do not mash the scrub brush into the carpet; brush over the top of the plaster to loosen it.
Vacuum the loose plaster off the floor with a vacuum cleaner, and repeat the process until no more plaster particles can be freed from the carpet nap.
Fill a bucket halfway with warm water, and add two squirts of dish soap.
Wash the affected areas of the carpet with the soapy water and a cleaning rag. Blot the rag over the affected areas, and rinse it often in the soapy water. Continue until no traces of the plaster can be seen in the carpet. Let the carpet dry for an hour and vacuum.