The accounts page lists all your Polkadot accounts - whether it was created directly on Polkadot-JS UI, created or added in the extension, and accounts on Ledger devices and Parity Signer added to Polkadot-JS UI. 

1. One way you can see your address is by clicking on your account name. This will open a pop-up on the right, showing your DOT address, account name and balance, and you can copy the address from there.

However, there's a simpler solution. Every address has its own unique visual identity in the form of a DOTs icon.

2. To copy your address, you can just click on this icon. A green plus sign will appear as you hover over it to let you know you're in the right place. (Simply click. No right-click necessary.) 

Once copied, you can paste it into a text file, a block explorer, or anywhere else you want to view it.