Go to the My Ads tab and move to the My Campaigns section then press on [ + Import Creative ] button on the campaign you would like to import creative to.
If you haven't created a campaign yet then press on [+Add campaign] button first.
You can learn how to add a new campaign here.
In the next form choose the ad network name you want to import creative from. Click on 'Ad networks' drop-down list to see the list of linked ad networks:
If you have not Linked the needed Ad Network at this stage yet, you can still do it when importing creatives:
Choose Ad Network you have an account with from the drop-down list and, just fill in correspondent credentials to import creatives from:
Click on it and you will see the full creatives list for the selected network:
Choose one creative and click Next. All fields will be filled automatically.
That's it, scroll down the page and click on the Save button:
If Ad Network doesn't support auto-import you will be notified about it:
No worries. You may just import creative manually. You will have to go to the ad network panel, find your ad zone and its ID and copy/paste them into the form fields.
How to find ad zone code/link url and zone_id read in Ad networks manuals