Datapump doesn't work on 11g XE on Windows (didn't try on Linux), you get something like that:
Export: Release 188.8.131.52.0 - Beta on Wed Aug 31 22:30:25 2011
Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved.
Connected to: Oracle Database 11g Express Edition Release 184.108.40.206.0 - Beta
ORA-39006: internal error
ORA-39213: Metadata processing is not available
It occurs because there are a entire directory missing:
If you copy that directory from another 11gR2 installation (even a Linux one) to your 11g XE and execute the following as sys:
SQL> execute dbms_metadata_util.load_stylesheets
you can make it work.