Hi:
If you don't already have Google Chat set up, you need to do so. Download the Google Voice and Video web browser plugin here:
https://www.google.com/tools/dlpage/hangoutpluginInstall the plugin. If you are using Firefox or Safari browser, then you must "activate" or approve the browser to run the plugin, for security reasons.
Sign into Gmail, not Google Voice. Look for your Chat avatar, midway down the left side of the Gmail inbox page. It's either a generic head and shoulders or a photo you may have uploaded. Click on it to make a phone call. That will add the Chat forwarding destination to your Google Voice settings page.