.EU and .DE domains register their nameservers differently then most other domains. To do this, you do not actually use the regular “Register a nameserver tool” in ENOM.
Example Domain: domain.eu
Example IP Address: 192.168.1.1
Once logged into your enom account, click on domains then My Domains. On this page, search for the domain in question and click on the domain name.
Next click on DNS Server Settings. This is normally where you would just point a domain to a new set of nameservers. With .EU and .DE domains, you basically are doing just that to register the nameservers. However, instead of simply entering ns1.domain.eu and ns2.domain.eu, you enter them like this.
Once you have entered them like this the nameservers will be registered, and you can now use them for other domains as well.