The Layout Chooser allows you to browse, preview and select Theme Builder layouts...
If you want to choose a layout for a space, you'll need to be a Space Administrator for that space, or a Site Administrator. Go in to Space Admin and choose the Themes link from the navigation bar.
If you want to choose a layout for the site, you'll need to have Site Administrator privileges. Go in to the Administration Console, and select the Themes link from the navigation bar.
Select Theme Builder as the theme for the site/space and then click the "Configure Theme" link.
You'll see a list of available layouts in the "Choose Layout..." panel on the left - simply click on a layout to preview it in the tabs on the right.
If you have sufficient privileges you'll also see a link above the Layout Chooser which allows you to access the Layout Manager (the tool that is used to add and edit layouts) and possibly also a link for Theme Administration (Site Administrators only).
This tab provides a quick overview of the layout, showing it's Layout ID, name and description:
It's always worth reading the description as it may contain important usage guidelines or information on how you can modify aspects of the layout such as navigation and colours.
This tab gives a very rough preview of the layout and is useful of your site has lots of layouts and you want to quickly browse through them:
This tab shows you exactly what things will look if you choose the selected layout:
Once you're happy with the choice of layout, click the "Save" button at the top of the Layout Chooser.
If you want to continue using the previous layout, just click "Cancel".
If you get a warning message stating that you're not allowed to select the layout, it's because the person who designed the layout has restricted which user groups can choose it for their space design on the Permissions Tab in the Layout Manager.
In order to select the layout you'll either need to become a member of the user group or ask a Site Administrator to set the layout on your behalf.
If you can't see the link then a Site Administrator has restricted access to the Layout Manager and you'll have to ask them to add you to a group that has privileges to use it.
You need the Layout ID when using the use-layout macro or menulink macro (with a location of "use-layout") to either override the layout for a specific page or dynamically select a different layout for the end-user.