I have just signed up with Lightroom, mainly to print out digitally photographed black and white positives of black and white negatives but can find no way to do this. All the instructions assume you have colour photos to begin with.
It's hard to answer without knowing whether these are JPEGs or raw, but the principles are exactly the same as all of the instructions you'll have read online for color negs. You need to invert the tone curve (grab the top of the point curve and drag it to the top left corner, and pull the bottom point to the right corner) to make it a positive, and then remove any color cast. If it's not neutral, hit the B&W button to remove any leftover color.