I have taken a raw photo in black and white with a red digital filter effect on my canon 40d. Why is it that on a thumbnail on my Mac it is black and white and on the Canon DPP software it is black and white but it opens in colour in Photoshop Elements?
When you shoot in raw your camera's senosr captures and stores every light value with no processing of the image, hence when you open a raw file in Adobe Camera Raw (ACR) you will see all the colour data. However, because you selected in camera to shoot in black and white, your camera has processed the raw data into black and white to create a jpeg and this is what you're seeing when you open the file in the Canon DPP software and the thumbnail on your Mac.
In Elements, you will need to create a black and white layer mask to convert the image into monochrome. You'll find this will work much better than trying to convert it to greyscale in Image>Mode>Convert Colour Profile.
Not sure you mean "black and white layer mask" there, ianpinion.....
If it's a newer Elements (6 or 8 for a Mac), then the "Convert to Black and White" feature is probably the best bet, with a Gradient Map layer a close second..... but a Layer Mask is a way to 'knock a hole' in a layer to see the one below, nothing more.