How do you open a mansion in the level builder?

[brell]

I have been testing my custom mansions in the MM1 HD and have found a few issues, which propably can be fixed by making minor changes in the editor. I am sure that more situations like this will come up. A mansion builder that upgrades to the Lion will not be able to make these fixes until the editor has been upgraded. I'm sure we can live with that for a while and I hope Vern will focus on upgrading the editor a.s.a.p.
There is also the argument that the editor has from the beginning been a part of the game and should of course run smoothly regardless of the OS version.