These steps are for MT5 on desktop. Menus differ in the web and mobile versions.
Market Watch lists the instruments you can trade, with live buy and sell prices and the spread.
To add an instrument:
Click + at the bottom of the Market Watch list.
Start typing the instrument's name, for example EURUSD.
Pick it from the list that appears.
You can also right-click in Market Watch and choose Symbols to browse every instrument. Double-click one to show or hide it.
You can't hide an instrument while you have an open position, a pending order or a chart open for it.