For most journals on SpringerLink, there is a 'Sign-up for alerts' option on the journal's corresponding Volumes and Issues page:
However, for Springer Open Journals, the e-alerts sign-up option is located on the journal's homepage. If navigating from SpringerLink; such as from a Volumes and Issues page, first click on the journal's title:
Once you've landed on the journal's homepage, scroll down and click the link on the right side of the screen that references 'Sign-up for article alerts and news from this journal':
A sign-up page for the e-alerts then appears. Complete the requested details so you can receive e-alerts when they are sent by the journal.