If you've just re-installed Ember, and need to get your Library from iCloud. You will need to leave your Mac on and running (leave Ember open too). Eventually your Mac/iCloud should download your Library from iCloud, this may take a long time depending on the size of your Library.
If nothing has appeard within a few hours try rebooting your Mac, this something kicks iCloud into downloading the data.
We recommend using a local library only, do not use Dropbox Sync or iCloud.