The Massachusetts Lottery is preparing for challenges posed by the opening of the Plainridge Park slots parlor later this month, and the coming of full-fledged casinos in the near future, according to Treasurer Deborah Goldberg. During a Greater Boston Chamber of Commerce forum this week at the Renaissance Boston Waterfront Hotel, Goldberg, who oversees the Lottery, said the agency “cannot be complacent” and must go beyond just advertising to increase profits in the face of casino gambling.