The FR-18d has many parameters that you can edit to customize your sound, including the individual volumes of the right and left hand, the type of drum set, the metronome tempo and volume, the reverb, etc. You can find a list of these on page 41 of the manual.
Use the following procedure to access those parameters:
1) Get out your FR-18d manual (you will need it). If you don't have one, you can download one here:
http://roland.com/support/article/?q=manuals&p=FR-18D&id=59684225
2) Press and hold the [SET] register (it will start flashing slowly)
3) Use the picture of the keyboard on page 42 to select the key which corresponds to the parameter you want to edit. You can figure out which parameter number you need to press by looking at the list of parameters on page 43-46 of your manual. The number to the right of each parameter, surrounded by a black circle, is the parameter number. For example, if you want to change the orchestra/organ level parameter, you would press the button on your keyboard that says "4" on page 42 of your manual.
4) In step 3, you chose the parameter you wanted to edit. In this step, you will be able to actually edit the parameter. Use register 1, 2, 3, or 4 to select the edit you'd like to make. Each parameter has a list of the changes that each register will make (a value, often referring to volume or a certain amount of an effect). Press whichever register that corresponds to the value you'd like to select for the parameter selected in step 3.
5) Press the [SET] register twice to leave the FR-18 diatonic's Function mode.