Have you tried adding the proxies without username and password, so just the ip address and port number?
I don't have a SquidProxies account, but it can be that they use ip address authentication only, in that case you don't always get a username and password along with your proxies.
So just authenticate your ip address in your SquidProxies proxy dashboard and add the proxies to Mass Planner without the username and password, in the format of ip address : port number (w/o the spaces), i guess it should work.