To change a photo to bitmap, you must first change the mode to grayscale. Image > Mode > Grayscale. You'll get a little popup like the one below warning that Photoshop is dumping the image's color information. Go ahead and do this, but make sure you don't save over your original image.
After converting the image to grayscale, you should be able to change the mode to Bitmap.
P.S...for future Photoshop questions, you can always ask the folks over at the general Photoshop forum.