I am working on making a xml meta-meta model in UML. I want to create
an enumeration class in my UML diagram with two attributes: SimpleType
and ComplexType. The type of SimpleType attribute can be one of the
primitive datatypes only i.e. string int boolean in my case. How do
restrict the attribute to only these primitive datatypes

Hey, I'm interested in this because I just finished an XML meta-model in
UML yesterday!

I did mine based on a very limited knowledge of XML and only two XSDs from
a corporate project using the XSD guidelines for our custom corporate
XML-based data store.  I also have limited knowledge of UML, being by
background a DBA for relational DBMS.

My UML has a supertype of Type, with subtypes Namespace-Defined Types and
Locally-Defined Types.  Then I break down the Locally-Defined Types as
Simple and Complex.  Then I establish relationships of the types to other
classes, such as Attributes and Elements.  I have an Enumeration class but
hadn't related it to the Types because my reference docs had no examples of
that.

Care to share your meta-model?  What tool did you use?