From 2.1.4, dictionaries are synced to the reader like the e-books. If you select to uninstall one, the dictionary file will be deleted from the device. Selecting to (re-)install it will "flag" the name of it to install the necessary file at the next sync with the desktop software (or wifi).
If an installed dictionary does not show up (no dotted line or back arrow etc), that means (Surprise! :-)) some database inconsistency, a sign out-sign in procedure should solve this.