I got a way to connect my D-Link DWM-156 modem on Ubuntu14.04LTS 64-bit. The problem was this Modem has a 32-bit driver and network manager identify it but network manager is unable to connect. Here's a link that will help you to enable dlink modem
. Actually I found a solution on help.ubuntu.com/community/
.It has suggested a way to enable modem using pppconfig & pon/poff and I tried this and as expected it worked. Now my modem is enabled and its working. To make my modem work I need to use terminal to enter command like pon - to enable modem, poff- to disconnect or to disable modem and plog - to check log. Here's the steps that I used and I want to mention it clearly that It is mentioned on the website help.ubuntu.com/community/ and so I am just copying it to here to help you.......
This is a command line based option to manage dialup connections. This makes it very flexible, but maybe not so straightforward to configure.
Collecting Information
You will need:
Your ISP's phone number
Your username and password on the ISP
The name of the modem device (/dev/...)
Setting up ppp
Open a terminal (Applications > System Tools > Terminal) and type
$ sudo pppconfig
You will be on the main menu. Choose 'Create Create a connection'.
Leave the name as 'provider', hit 'Ok'.
Select 'Dynamic Use dynamic DNS', hit 'Ok'.
Select 'PAP Peer Authentication Protocol', hit 'Ok'.
Enter your user name for the ISP, hit 'Ok'.
Enter your password for the ISP, hit 'Ok'.
Leave the speed at 115200 as recommended, hit 'Ok'.
Choose Tone or Pulse dialing, hit 'Ok'.
Enter the phone number to your ISP (do not use any dashes), hit 'Ok'.
You can try to have your modem detected automatically, but it did not work for me, even on my easily detectable external modem.
If the modem wasn't detected, it will ask you for the port your modem is on. Enter the device name for your modem, hit 'Ok'.
A summary screen will appear and give you the opportunity to make changes if needed.
Choose 'Finished Write files and return to main menu.'.
Choose 'Quit Exit this utility'.
Exit the terminal window, type:
$ exit
Connecting to the ISP
Log out and back in (for the group setting to take effect). Every "dialout/dip" user will now be able to connect and disconnect with these commands:
$ pon # connect to the ISP configured as "provider"
$ poff # disconnect the ISP configured as "provider"
You can also check the last messages generated by these commands with:
$ plog # shows the (status) messages generated by pon and poff