libqt4-dev installation

[ahaider]

I am trying to execute

apt-get install libqt4-dev

The response is

Reading package lists... Done
Building dependency tree
Reading state information... Done
Package libqt4-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package libqt4-dev has no installation candidate

Whats the problem ???

[smoker]

i had a look at synaptic package manager, and i can install this ok, i have all the repositories enabled, except 'source' maybe you need to enable other repositories?

[ahaider]

i had a look at synaptic package manager, and i can install this ok, i have all the repositories enabled, except 'source' maybe you need to enable other repositories?

how to enable other repositories. by modifying /etc/apt/sources.lst, or by some other way ?