I'm not sure if this is how you are really defining your $scope.onAddInterface
function or if it is just an example... Nevertheless you should be doing it like this:
$scope.onAddInterface = function() {
$scope.showCreateNewInterfacePanel = true;
}
update
Also make sure the link and the collapsible element are under the same $scope/Controller:
<div ng-controller="Ctrl">
...
<a ng-click="onAddInterface()">add interface</a>
...
<div class="collapse" ng-class="{in:showCreateNewInterfacePanel}"><div>
...
</div>
Controller:
function Ctrl($scope) {
$scope.onAddInterface = function() {
$scope.showCreateNewInterfacePanel = true;
}
}