If you are using MIDI OX you should see the following window:

You’ll want to use the drop down window to select the desired parameter (as referenced in the documentation) and then hit the Send button.

Notice the text by the numbers which are simply part of the standard MIDI spec.  These descriptors are irrelevant for the DT amps – again, just look for the number that correlates to the DT parameter you want to send.  It also may be confusing that the numbers are not listed sequentially.

If you wanted to change the preamp for the current Voicing of channel A you would select number 11 (labeled expression in the MIDI OX drop down) and number 3 (if you wanted to load the Hiway 100 model for example) using the up/down buttons in the field labeled Value.  Hit send and the current Voicing on Channel A should be the Hiway 100.

Here are some links to the MIDI documentation:

http://line6.com/media/dt50/DT50_MIDI_CC_Reference_v2_0_RevA_English.pdf

http://line6.com/media/dt50/DT25_MIDI_CC_Reference_v2_0_RevA_English.pdf

http://line6.com/media/dt50/DT_MIDIGuide_v2_0_RevA_English.pdf