Once you have logged into your wallet, click on the Fund title.
Now click on the Gateways heading.
To add a new Trust-line, click on Connect a Gateway.
Copy and paste in the Ripple address of the Gateway that you wish to trust for an asset that they offer. Set a limit of how much of that asset you are willing to hold up to. In this example, I trusted the Streets of Silver Gateway address rVSB2uzC9cjKfoMQdpQW78qUdSkdHiU4y for 10 Ether.
A popup will ask you to confirm your choice, click on Confirm.
Wait 3-5 seconds and your client should report that your Trust-line has been created.