When adding a service from your ArcGIS Server, do it from the My Content window and not from the Map. In My content, create a folder to organize your content.
- Click the Add Item pulldown
- Select the option From the web
In the next dialog box, enter the URL of Map Service layer from your ArcGIS Server Rest Services Directory. If the service is a secure service, the dialog box will change and allow you enter your credentials. You will also have the opportunity to store the credentials with the item so your ArcGIS Online users will not get prompted for credentials every time the service is accessed. The option is yours.
Once the item is added. It will show up as a feature layer in ArcGIS Online.