Hi,
Thank you for trying MyEclipse 2015 CI 10. Sorry that you are seeing this issue.
I could configure Tomcat 8.x without any issues at my end.
Once you add the Tomcat 8.x to the Server Runtime Environment, you should select it in the New Server wizard to complete configuring the Tomcat 8.x server.
Here is the screenshot for your reference :
If you are still seeing issues, please send us screenshots of the steps you are following to help us get a visual and investigate further.
Let us know how it works for you.