what did you google?

i think i might know what's confusing you though

you know when you assign a preset bump map and that dialog comes up wtih all those options?

and you click one of those options and it assigns the bump to the material?
click on the button that says 'file'
then you'll get the option to change the bump depth and whether you want to use it as a bump map or normal map

hit the button next to Bump Value slider (the one with a black arrow going into a square) and you'll be taken to a palce where you can load a file

a text field with the label Image Name next to it and a button with a file icon is there. click the file icon and locate your bump map (if you set up your project using default directories you'll instantly be taken to the source images folder or the folder that was last seen when you saved out an image in maya)

you can get back to the other dialogs by hitting the two buttons next to Focus and Presets (the ones with black arrows on the squares)

hope that answers your question