Open Desktop Manager, click Backup, and make a complete back of your BB.
If you don't already have your current OS downloaded to your PC... do so (making sure you get the exact same OS as currently running on your BB: Latest Operating System Available For Each Blackberry Device
Execute the file.
Open Desktop Manager. Click Application Loader, click ADD, you should see a list of options and apps loaded on your BB, and on your Desktop. In the BB column, find the Memo App, the box next to it will likely be unchecked, so check it. Proceed as instructed by the Application Loader.