How recent is the list, because proxys change from day to day it seems. (they go up and down almost as fast a "Warez" sites it) Find the button or option in Multi Proxy to Check All Proxies and let it do its thing, which might take a while. Then check the list and see which ones are slow, responding or dead and prune away some of the dead ones and try replacing them with new untested proxys from a current list then run the check again.

I just thought of something else, do you have a firewall going? because i remember Outpost giving me problems when i was messing with MultiProxy. Try turning your firewall off temporarily and testing Multi Proxy. Hope some of this helped.