I ran in the same issue but now it is resolved. It seems the problem is that the plugin was designed for import into buddypress forums, not bbpress ones. So you must activate and configure buddypress forums and the plugin will work out of the box (assuming your json files are correct).
1)Go to plugins->installed plugins->buddypress->settings->Components->retired and activate group forums (legacy). Save settings, a "Forums" tab appears.
2)Click Forums tab, in the left side, click on Install Group Forums.
3)Then you'll need to create a new page or use an existent one in order to assign "Discussion Forums" to it (in Buddypress Settings->Pages). Save settings and should it appear a message bar standing for generate config file or repair or something similar (I've a fish memory, sorry), click it. It will fix your config file path in buddypress->settings->settings tab->legacy group forums.
And that's all, now it will work or so did it for me.
In case you want to use bbpress forums, you can now import data to bbpress from buddypress forums in tools->forums->import forums from bbpress1. (You can follow the guide in http://codex.buddypress.org/getting-started/guides/migrating-from-old-forums-to-bbpress-2/ )
Hope it helps.