If you are unable to find a particular Pair/Instrument on MetaTrader, you will need to select Show all in the Market Watch section.
Right-click on the Pair/Instrument section.
Choose Show All.
If you are on a mobile/tablet:
To be able to add more Pairs/Instrument on a mobile device, you will need to click on the + icon on the top right-hand side of your screen as shown below.
You will need to choose the appropriate category.
Then you will need to choose the pair/instrument you want to see.