Does your dictionary show up, when you click Archived Items in the Kindle for PC library? If so try re-downloading it by double-clicking the dictionary cover.
If if doesn't show up as an archived item, you'll have to remove its DRM and manually copy it to the My Kindle Content folder. If you want to use it as a lookup dictionary you'll also have to change its ASIN, because the Kindle for PC app doesn't support third-party dictionaries as lookup dictionaries. See this
thread for more details.