I was digging through the link Pat sent in the comment:
bugzilla.xamarin.com/show_bug.cgi?id=43514
And found a helpfull comment by Pavel Sich, he said:
Just make sure you enable the keychain access in Entitlements and select the entitlements for Simulator (Debug) builds too. By default this is not set.
In my xamarin solution, I double clicked the .IOS project to open the options pane, selected IOS Bundle Signing and changed the Platform select box from “iPhone” to “iPhoneSimulator”, then set the field Custom Entitlements to my Entitlements.plist. Now it’s working fine for me.
Just a note, this is after editing the Entitlement.plist as suggested by Thibault D. in the previous answer.
Hope this helps.