Normal registration emails should be sending though (e.g. Welcome email should be sent when approved). If you're needing an entirely separate custom email though that's doable using CB Auto Actions. The below should work.

Global
Triggers: onAfterUserApproval
Type: Email
User: Automatic
Access: Everybody
Conditions
Field: Custom > Value
Custom Value: [var2]
Operator: Equal To
Value: 1

That should send an email of whatever you like to whomever you like when a user is approved. The user object for the auto action will be the user being approved so if you add [username] to the email body for example it'll be the username of the user that was approved.

If you want to send an email when a plan becomes active you can do so using the emails configured under the Presentation tab of your plans or the CBSubs Email integration for custom recipients. That would be easier to configure at least if this is meant to be entirely tied to a CBSubs plan.

You won't be able to use normal CBSubs substitutions like that in it since it doesn't pass through CBSubs. You'll have to use the substitutions provided by the CBSubs system plugin for content plugin usages.

[cbsubs:subscriptions plan="PLAN_ID_HERE" output="expiry_date" /]

See the description of "cbpaidsubsbot" in System > Extensions > Plugins for further usage information. Note you'll need to toggle in "Content Plugins" under the Parameters tab of your auto action for these to work.