If you get the following error while using dj-catalog2,
 

Fatal error: Call to undefined function curl_init() in C:\wamp\www\...\administrator\components\com_djcatalog2\lib\djlicense.php on line 36

it means that you have not cURL module (library) on your server.
Usually it is enabled by default on your server, otherwise have to enable it to make dj-catalog2 work correctly. 

The above error refferes to curl_init() function that initializes a new session.

The cURL installation process depends on the server, so there is no one recipe how to do it. Have to ask google how it install it on the server you're using :)

E.g. if you work on localhost on Windows, you can do it manually by copying the proper *.dll files to your system folder.
On the other hand, on live servers, you should find it in cpanel. If you can't find it simply contact your hosting provider.