When you add your accounts to the dapp, most tokens will be detected automatically and will be reflected in the list of assets on the portfolio page.
If you hold a token in one of your accounts and it doesn't automatically show up in your portfolio, you can add it manually.
To start this process, find hit the three vertical dots next to the 'Assets' heading:
This will bring up a menu that lets you search for the token:
Input its name and click on it, and then follow the prompts to add it to your assets list.
You won't be able to add a token unless you hold it in one of your connected accounts.