You can import tasks from Trello 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 "Trello" in the list of import options and click "Upload your .json file" (see below on how to export your trello data into a .json file). Select your file and confirm.
If your Trello board contains Team members assigned to cards, only the first member listed will be taken into account as Planless takes maximum one assignee per task.
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 export a Trello board to a .json file
Inside of your Trello board, go to "Show menu" on the top right and select the "...More" option. Click then the "Print and Export" menu and select "Export as JSON".
Save the .json file to your computer.