A new volume can be loaded via File > Open data. The progress bar in the bottom right corner indicates how long the loading process will take.
The file must contain a head of 6 bytes followed by the actual volume data. The header contains the size of the volume in x, y and z direction. The volume itself must be stored with 16 bit per voxel and the data has to be given in a cartesian grid. There may occur an error if a file doesn't meet this requirements.