Singer Dean Martin opened to a capacity audience on the Las Vegas Strip Thursday night, his first stage appearance since dropping out of a national tour with Frank Sinatra and Sammy Davis Jr., the old Hollywood "Rat Pack." The tour opened March 13 in Oakland, Calif., but Martin was forced to leave the road March 21 when he was admitted to Cedars-Sinai Medical Center in Los Angeles suffering from a flare-up of an old kidney ailment. The 70-year-old entertainer was flown to the California hospital from Chicago where the "Rat Pack" had performed the previous night. "Frank sent me a kidney, but I didn't know whose it was," Martin told first nighters at Bally's Hotel-Casino where he is booked for a one-week appearance.