To setup your automation by optin date you will need to be in your Automations area. Go to Campaigns - Automations from the Top menu
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Once you are in Automations you will either be creating a New Automation if you don't have one already existing or you will be editing an existing Automation.
Note: In order to edit an existing Automation you will need to disable the Automation from the Automation Menu Options. 

I'll start with an existing automation. Select your automation and click Disable Selected. Once you do this you will see the status change from Enabled to Disabled as shown below.
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 Next Double Click your Automation to enter it.
Once you have entered your Automation you will want to click on the Top Condition which says "Start Campaign".
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This will then show you a popup that gives you two options. One for your list name and one for the Added Date or Optin Date.
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You simply select the drop down for Opt-in date and this allows you to now send email messages based on Opt-in date vs Added Date.
Read more here about the difference between optin date and added date. 

If you have a new Automation your steps will be a little different. Once you are in the Automations area you will select Create New Automation. 

Once you click "Create New Automation" you will see the option for Subscribe or List.

You can select either On Subscribe or On List Name as both options have the option for Optin Date. To learn more about the differences between the On Subscribe and On List name Click Here 

Once you make your selection you will see a popup that gives you two options. One for your list name and one for the Added Date or Optin Date.  

You simply select the drop down for Opt-in date and this allows you to now send email messages based on Opt-in date vs Added Date.
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Read more here about the difference between optin date and added date. 

That's it! That was pretty simple wasn't it? Let us know if you have any questions or comments.  
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