The free version of Mindmeister only supports a limited number of export formats. With a little workaround though, you can convert these mind maps easily to OPML.
Let’s take the highly popular, and free, How to Win Friends & Influence People by Dale Carnegie as example.
Go to the Export dialog and select Document Outline as format. A Rich-Text Format document (.rtf) will be downloaded.
Now open the document and press ⇧⌘T (Make Plain Text). We have to do some manual work now. With the original mind map still open. Select the lines of text where a subnode will be and indent them. It is easier to search for the “headlines” and just select the lines between them. Make sure that only the main node isn’t indented.
Now select everything and press copy.
Go to MindNode. Open a new document and make sure that nothing is selected. Even better, delete everything so that the canvas is blank. Then type paste. If you had empty lines they will shop up as empty nodes. Just delete them.
Reorder everything as needed.
Now go to Export and select OPML.