Warren Robbins, the founder and head of the Museum of Africa Art, arrived at Bloomingdale's at Tysons Corner Saturday night decked out in a patterned tuxedo jacket but holding onto his trousers. In minutes, with the help of a store executive, he was in the belt department.

Robbins said he had simply forgotten his belt. "It wasn't dangerous, only disconcerting."

That was the only hitch in the Museum of Africa Art's benefit planned to kick off the museum's fund drive to match a $225,000-grant from the National Endowment for the Humanities. According to Jonna Lazarus, chairman of the event, by benefit time $25,000 already had been collected, and sure that the figure would be at least $30,000 by the end of the evening.

The benefit turned out a mix of political, arts and social types including Nancy Hanks, who said that her plans when she steps down next month as head of the National Endowment for the Arts go only as far as a rest in North Calorina. "But I plan to keep my house in Washington, and spend much time here," she added.

Robbins and Bloomingdale's had tapped several supporters of the museum, the only museum in the country dedicated entirely to African art, as honored guests with model rooms designed in their honor. Several of the special guests were represented by brothers and sisters.

Sen. Hubert H. Humphrey (D-minn), the museum's first chairman of the board was in his home state recuperating from his recent operation. His sister, Frances Howard, who spoke with him Friday, said, "he felt better that day than he had any day since the operationBrothers of "Roots" author Alex Haley and television, newsman Mike Wallace were on hand to represent them.

Entertainer Dick Gregory, who drove down from Plymouth, Mass, with his daughter Lynn, 16, had asked the store to make "his" room a kitchen, "so I could make the point strongly about eating health foods," but quickly settled into the chrome and glass desk in his model room and talked with guests.

Gregory gives close to 300 college lectures yearly. "The questions from the kids are far more spiritual today than even three years ago," said Gregory. "No political questions at all. Spiritually transcends politics and religion," he said.

Sen. S. I. Hayakawa (R-Calif) toured the model rooms, praising the African art displayed in each. A collector of African art, he discovered it in California 25 years ago eith long-time friend Warren Robbins. "He came to it from German expressionism, and I got to it through French post-impressionism and cubism," said Hayakawa.

The senator was finally directed to his room, a corner of a major display area where his stripped beret, a photograph of his mother and a baseball bat of his were among the room's decorations. Glancing at the shiny fabric-covered chairs, he said, "Pretty, but a little ornate for me."

The Museum of Africa Art drum Ensemble, most of whom are members of the museum's staff, provided the music for Coty Award-winner Mary McFadden's designs, led on stage by Olufnmilaya, a member of the ensemble. McFadden, who lived in Africa for five years and founded a workshop to develop tribal sculpture in Inyange Province in Rhodesia, often used her own art collection as backdrops for her fashion shows.

McFadden's clothes this season are narrower in shape than in past years "just because everyone else was making very full clothes." Still very clearly McFadded signatures are the handpainted silks, this year in Oriental-inspired patterns: the tight Maarli pleating, the quilted coats and the bold, gold jewelry slung over the shoulder.About one model tied bondage-style with beaten brass disks worn as breastplates, McFadden said, "It's just an esthetic way to show jewelry."

The food for the evening, like everything else, had an African theme. Waiters, in dashikis from Toast and Strawberries, were touting the meat as elephant. "Not so," admitted Mike Waters, who provided the food. "Elephants are endangered speciesso our meat is really steamship round of beef."