RIM has apparently discontinued download and support for all versions of it's BlackBerry Desktop Software version previous to version 6.1, currently available at
this link.
I've archived previous versions here if anyone needs:
File name Desktop Manager 6 Latest working version >>
610_b034_multilanguage BlackBerry_Desktop_Manager_6.1_b20.exe 600_b047_multilanguage.exe Desktop Manager 5 501_b080_multilanguage.exe 501_b080_multilanguage.exe 501_b073_english_nomediamanager.exe 5_1_.0.0_Release021_multilanguage.exe Desktop Manager 4.2 421_b017_english.exe Desktop_Software_v4.2_SP2_(English).exe