Brewer said he "couldn't see anything upstairs." That can mean it was too dark to tell whether Oswald was there or not. All it means is that they didn't see Oswald anywhere - upstairs or down in a dark theater. But they knew he had gone in and somehow got past Burroughs without being noticed.
How did they know that he had gone in? Neither Brewer, Postal, or Burroughs saw anyone go in at that time, and Brewer and Burroughs did not see him when they checked the balcony and the theater area.