In order to show remarketing ads, you'll need to set up a separate campaign in AdWords. This campaign should target the Google Display network (rather than Google Search) and will not require keywords, but should be built around audiences you'll add to individual ad groups.
Before setting up a remarketing campaign, you'll need to:
Add the remarketing code to your website
Create remarketing audiences for visitors you'd like to target
To create a remarketing campaign, follow these steps:
Click on the Campaigns tab.
Click on + New campaign to create a new campaign.
Choose Display Network only.
Name your campaign and pick your locations and languages.
Under Bidding and budget in your campaign settings page, choose I'll manually set my bids for clicks and enter your budget.
Click on Save and continue.
Next, create your first ad group. Your remarketing campaign should have separate ad groups for your different remarketing audiences. For example...