Here's the documentation: https://sawtoothsoftware.com/help/lighthouse-studio/manual/index.html?id_smrt_importingutilityruns.html
Here's my quick summary:
1) First import your .HBU file into the market simulator, so you can use HB utilities. First do that by establishing a new project name (type CBC) in SMRT and clicking Analysis + Run Manager + Import (and browsing to the .HBU file).
2) And, if you additionally want to import the .CHO file into SMRT for running Counting analysis within SMRT, then after importing your .HBU file, you additionally (within SMRT) click File + Import (and then browse to your .CHO file).