You open raster layers through raster manager.
Select your file and then Attach.
Unlike with a vector file, you cannot import this directly into the project.
MicroStation automatically shows the raster overtop of the LiDAR points. If you would like the LiDAR points to be displayed on top go to TerraScan Settings and change the Draw as: setting to Raster (from Point) to view the raster instead of the point cloud.
Don’t forget to update your view for the change to take affect.