If someone creates a partition for ApacheDS and does not add a context entry in the configuration of the server.xml (possible with ApacheDS 1.5.4 and above), it can't be used. A context entry has to be added first with the help of an LDAP add command.
Unfortunately, this is not possible with Apache Directory Studio 1.2.0, id the DN of the suffix contains more than one RDN. For instance, New Entry does not work for the entry dc=example,dc=com
I recommend a new menu item called "Add new context entry" or "Add new suffix", which is comparable to the Add entry dialog, but allows the user to provide the complete DN without a parent.
A highlight would be, if the dialog detects possible values from the namingContexts attribute from the Root DSE. If a new partition has been added to the server.xml, it is listed there. See here for details: