Right-click on the "Instrument | Sell | Buy" row in the "Dealing Rates" window. Select "Preferences" and then "Settings".
You will then have a selection box as per the below where you can move "Invisible Instruments" to the "Visible Instruments" column by clicking on the ">" button. This will add them to the Dealing Rates window.
Find the Share, Index or Currency you wish to add in the "Instruments" window and Right-click on it. Select "Add into Dealing Rates" from the pop-up menu. An example of adding EUR/CAD is shown below:
When you have your Dealing Prices window customized as desired, be sure to select "File", "Save Settings" before you exit.