If you don't have the anatomy available for your subjects and just want to process the signals, select "Default anatomy: YES" when creating your subject.
Then if you import a segment of recordings, apply a bipolar montage and display it, you should not have any problem.
This would work only if the naming of the channels follows the same logic as in the example below, with some letters (the electrode name) followed by the index of the contact on this electrode. A bipolar measure can be taken only if there are two adjacent indices.
If this is not the case: you may either rename your channels from the channel editor, or edit your own custom montage:
https://neuroimage.usc.edu/brainstorm/Tutorials/MontageEditor
