discuss.answer
Tried to add a Menu for FormIt and it said it already existed. Looked at the menus table and it had the parent at components. I don't have a menu called components. Changed it to Extras and logged out and back in and now it is there.
Extras are additional tools that extend Modx's functionality. I may be interpreting your words wrong, sorry if so.
If its successful then you are probably installing successfully. The error to me looks very minor, but I can't be sure. It will be because of one of the Extras I guess.
I haven't used formit myself much but if it said the menu existed it was probably the filename you chose, each one will need a unique filename, could be the default filename so already has a file by that name.
For the parents at component you can customize that, I believe.
