To enable spell checking, select one of the supported languages using pop-up menu→languages→
[your language]. from any editor.
Some spell checker options can be configured from Preferences->Behavior->Spell checker options.
User can add his personal words for a particular language from the context menu on word selection. The first time user adds a custom word, a user dictionary file is automatically generated in
You can install another dictionaries copying them into <freeplane-install>/resources/ortho. The dictionary files dictionary_XX.ortho converted from aspell are available on http://sourceforge.net/projects/freeplane/files/spell_check_dictionaries/ .