Please see http://www.whmcs.com/iphoneapp.php for details.
1. To begin with (if you haven't already done so), you will need to order the iPhone addon from our client area. It's available as an addon to a license and can be ordered by logging into our client area @ www.whmcs.com/members/clientarea.php and selecting "View Available Addons" from the navigation bar.
2. Once you've ordered and paid, the addon will be instantly activated and you'll just need to perform a local key update by going to Help > Check for Updates inside your WHMCS installation to have it take effect.
3. Now, you're ready to download the iWHMCS app from the Apple iTunes App Store which you can do by simply searching for iWHMCS in iTunes or using this link to take you directly there: http://itunes.apple.com/gb/app/iwhmcs/id343647658?mt=8
4. Once downloaded, simply sync your phone to install the app as normal
5. Now locate and open the app on your iphone's home screen
6. The first time you start it, you'll be asked to enter your authentication details. These consist of your WHMCS URL, admin username & password and an access key.
7. The WHMCS URL should be the URL to the frontend (client area), for example "http://demo.whmcs.com/" or "http://www.yourdomain.com/whmcs/" and the admin username & password the same as you use to login to the main admin area.
8. The access key is slightly more complicated. For this, you need to open up the configuration.php file for your WHMCS installation and add a line to it such as the one below:
$api_access_key = "abc123";
The abc123 in the above should be replaced with a random series of letters and numbers created by you, and that same value is what you'll need to enter into the Access Key field of the iWHMCS configuration screen.
9. Then simply click Save and the connection will be tested. If any of the details were entered incorrectly you'll receive an Access Denied (username/password failure) or Invalid Access Key error message and be returned to re-enter the details.
10. If all was correct, you'll be taken straight to the homepage overview.
The common error responses from the API can be found at API:Responses
This error means the API Access Key has not been added successfully to the configuration.php file. Please refer to step 8 above. The $api_access_key line should go before the closing ?> tag.