To add photos to Arkiver from your Picasa account, you'll need to connect to Picasa.
1) Click "Connect to Picasa" in the Moment Uploader screen
2) Sign In with your Google Account
3) Click "Accept" to allow Filepicker (the service Arkiver uses to connect to your online accounts) to access your Picasa files.
3) That's it! You'll be connected and all your Picasa albums and you can navigate through them and pick photos to upload to your Arkiver. Once you've connected, you won't have to go through this process again. Hooray!