IIRC, the supplied sample XML files haven't been given a thorough vetting for a while, and they may be slightly out of date; in any case, they don't necessarily apply to any one journal's configuration. I haven't seen the kind of error you are encountering with the users XML import plugin; to narrow possible problems, I would recommend the following:
- review http://pkp.sfu.ca/wiki/index.php/Import ... ML_Plugin;
- start with a subset of the 100 users as a test-case, say 5 or 10 in one XML file;
- pare the import file down to the essentials (for example, don't include biography elements if you don't have biographical information for the users);
- check that your import file validates;
- report back with problems if you still hit them.
If you still encounter problems, I'm happy to review a sample of the XML file. You can either post part here (obfuscated emails/usernames, of course) or PM me with more details.