You can import tasks from Asana by clicking the "Import tasks" button on the top of your tasks or by right clicking on a task and selecting the "Import tasks" option.
Select then "Asana" in the list of import options and enter your Personal Access Token from Asana and click "Connect to Asana".
Once done, you will be asked to select your Asana's projects you want to import. (maximum 100 projects in 1 import). Select the projects you want to import and continue.
If your Asana projects contain Team members assigned to tasks, it will be asked to you to match them to existing team members, create new ones or ignore them.
Once done, just select the destination of your import and finish your import.
How to get your Personal Access Token in Asana
Inside of Asana, go to your "My Profile Settings" (under your avatar on the top right corner) and then to the "Apps" tab.
Click on the bottom of this window the "Manage Developer Apps" link.
Once there, you'll find a "New access token" button at the bottom of the "Personal access tokens" section.
Click it, name your token and accept Asana Api terms and click the "Create token" button.
Copy your token and paste it in Planless.