the worflow to use helper frames changed a bit.
Basically you start by placing your 2D tracks on the footage.
Once they are tracked, go to image sequence on the tree, right click -> import and select the helper frames you want to use.
Sleect all of them at the same time wiht Shift+click so that once imported in mmp, they share the same camera (if not you can always reassign images to a differetn camera later).
Modify you work area in the timeline so that it now includes those frames for solving.
Go to the view that shows multiple panels. Go to your helper frame 1 (will be at the end of the image sequence) and click on the the little icon on the top right corner to lock the image in time.
Go to another panel and set it to display your other helper frame and lock it in time.
Finally, go to your image sequence and play the tracks up to a point where you see common objects between the panels. Clikc on the 2D track in the image sequence panel and place it on the other 2 panels where you locked your helper frames.
Helper frames need a minimum of 12 points in common with the image sequence.
Next time you solve for the camera, helper frames points will also be reconstructed in 3D, your indication that it worked.
If not make sure you ahve enough markers, that the 3d space was well smapled and that you didn't do any mistake.
