Your Multisig address is displayed on the main dashboard when you log into Polkadot Multisig. You can also get to this screen by clicking on "Overview" in the left menu.
Your Multisig address is displayed in the "Vault Address" field. This can be copied easily using the copy button.
Also, if you hover over the address, you will be able to see additional information, including any identity that has been set on the address.