Make sure that the Settings.bundle directory is included at the top level of the project. By top-level, I mean that on the file system, the Settings.bundle directory should be in the same directory as your ProjectName.csproj file. In MonoDevelop, it will appear as one of the top-level folders under the project node.
Set the Root.plist Build Action to BundleResource (do not enable "Copy to Output Directory". Hint: if you find yourself setting "Copy to Output Directory", don't do it - it's almost always the wrong thing to do - we've actually discussed removing the option altogether.)
Now, for the plist file format - here's what it should look like:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>PreferenceSpecifiers</key>
<array>
<dict>
<key>Type</key>
<string>PSGroupSpecifier</string>
<key>Title</key>
<string>Group Name</string>
</dict>
<dict>
<key>Type</key>
<string>PSTextFieldSpecifier</string>
<key>Title</key>
<string>Username</string>
<key>AutocapitalizationType</key>
<string>None</string>
<key>AutocorrectionType</key>
<string>No</string>
<key>KeyboardType</key>
<string>Alphabet</string>
<key>IsSecure</key>
<false/>
<key>Key</key>
<string>usernameTextField</string>
<key>DefaultValue</key>
<string></string>
</dict>
</array>
<key>StringsTable</key>
<string>Root</string>
</dict>
</plist>
Looking at your Root.plist file, it appears that the PreferenceSpecifiers key is not in the top-level dict node (it needs to be). In other words:
get rid of the following lines at the top of the file:
<dict>
<key>Root</key>
...and one of the following lines at the end of your file:
</dict>
That should do it...
Now... all that said, I just recently figured this out myself while trying to add settings to my personal side project and decided that we, at Xamarin, need to make this easier. I've already fixed a few minor issues in the upcoming MonoDevelop 3.1, but I hope to get the time I need to add templates for Settings.bundle to MonoDevelop's "Add New File" wizard. I'd also like to add plist key/value hinting like we do for Info.plist and Entitlements.plist so that MonoDevelop's plist editor can better guide you when editing these plist files.