You need to include an ELEMENT and an ATTLIST:
<!ELEMENT myelement ( #CDATA | myotherelement )* >
<!ATTLIST myelement
myattr CDATA #IMPLIED
>
<!ELEMENT myotherelement EMPTY >
You also need to specify how the new element fits into XHTML by overriding a .content, .class or .mix entity, e.g.
<!ENTITY % Misc.class
"ins | del | script | noscript | myelement">