as Anthony said above really.

You could also use the stored procedures to do this via T-SQL instead of the GUI. On the primary use this to set the threshold

exec sp_change_log_shipping_primary_database @database = 'yourdb'

, @backup_threshold = 'backup_threshold in mins'

or disable the backup threshold alert using

exec sp_change_log_shipping_primary_database @database = 'yourdb'

, @threshold_alert_enabled = '1'

On the secondary up the threshold using

exec sp_change_log_shipping_secondary_database

@secondary_database = 'yourdb'

, @restore_threshold = 'restore_threshold in mins'

or disable using

exec sp_change_log_shipping_secondary_database

@secondary_database = 'yourdb'

, @threshold_alert_enabled = '1'