To add a link you just saved to a bundle, select 'Add to a bundle' from the save pop-up. This will display all of your bundles and you can check off what bundle you'd like to add your link to, or press the small '+New' button on the top right of the bundle drop down to create a new bundle to add your link to.
To bundle links that you've previously saved, go to your bitmark page, located under the 'Your Stuff' tab when signed in. Hover your cursor over the link you'd like to add to a bundle. From there, click on 'Add to bundle'. If it’s a new bundle, you’ll want to name your bundle. Or you can add your link to an already existing bundle.