DataImportHandler should work for full imports, but due to the way DSE manages Solr configuration, it doesn't support incremental delta-updates at the moment.
When you hit the full-import button in dataimport.jsp, you should see the text in the right hand frame update when the import is complete. The frame is just rendering the raw response from solr, so it should contain XML data looking something like:
<?xml version="1.0" encoding="UTF-8"?>
<response>
<lst name="responseHeader">
<int name="status">0</int>
<int name="QTime">6</int>
</lst>
<lst name="initArgs">
<lst name="defaults">
<str name="config">dataconfig.xml</str>
</lst>
</lst>
<str name="command">full-import</str>
<str name="status">idle</str>
<str name="importResponse"/>
<lst name="statusMessages">
<str name="Total Requests made to DataSource">0</str>
<str name="Total Rows Fetched">2</str>
<str name="Total Documents Skipped">0</str>
<str name="Full Dump Started">2012-09-11 20:50:02</str>
<str name="">Indexing completed. Added/Updated: 2 documents. Deleted 0 documents.</str>
<str name="Committed">2012-09-11 20:50:04</str>
<str name="Optimized">2012-09-11 20:50:04</str>
<str name="error">Could not write property file. Delta imports will not work. Make sure your conf directory is writable</str>
<str name="Total Documents Processed">2</str>
<str name="Time taken ">0:0:2.132</str>
</lst>
<str name="WARNING">This response format is experimental. It is likely to change in the future.</str>
</response>
The documents should be imported into the index despite the warning about delta imports.
For more details, the tomcat log file is located in the /var/log/cassandra dir.