If you insist using your downloaded version, we have to know in what format you downloaded it...
=> RPM, tar.gz, ....? (if we want to help you...)
Remember, this can make your system unsupported, because of your system and configuration, we won't be able anymore to copy your situation.....
=> it won't be a 'normal' PCLinuxOS system anymore
And the application is not available here: http://portablelinuxapps.org/
Did you try the repository spout.ussg.indiana.edu - as suggested by Aradalf
=> that one should work (remember, only one repo at a time!)
- else (if urgently needed) I can send the needed PCLinuxOS-unetbootin-rpm(s) from my local repo, which you can put in /var/cache/apt/archives, and then install via Synaptic (which will be tomorrow morning; now 18u49 here, and going to music school and French lessons this evening - after that too late to get my external HD, and upload it here)