At least, to use apt-get from the command line you have to set
some environment variables:

export http_proxy=http://username:password@proxyserver.net:port/
export ftp_proxy=http://username:password@proxyserver.net:port/

now you can call apt-get update, apt-get install <somepackage>, etc.

But I don't know how to configure the appmanager to use this
settings.

regards nicolai