Please check your php configuration upload_max_filesize. Most likely you'll find this is set to 2M. DT Register is just a little bigger than that, so increase this parameter to something like 4M or larger and you should be all set. Contact your host if you can't make this change on your own. Thanks.
BTW...in the back end...go to HELP, System Info, PHP Information and look for "upload_max_filesize".
When that is changed...try it again. If you do the change yourself, you will have to restart Apache for the change to take effect. Until this says 4M or bigger, you won't be able to install.
You can also try using Joomla's Install From Directory option:
1. unzip the com_dtregister2_7_2e_J15.zip install package. You'll then have 3 folders and one .xml file.
2. via FTP, copy these folders AND file to /tmp/dtregister. Create this directory. Dump the unzipped package there.
3. go to the Joomla installer area and go to the middle section that says Install from Directory. Put in this directory location: /your_joomla_location/tmp/dtregister
4. Click the install button
This may work better for you. DT Register is just a big program.
After trying this...please submit a ticket if you have further issues.