I've been contacted about this as well, and sadly I cannot offer any advice besides what I was told by Skype via one of HB's guys. In essence, that's what they told me:
Heatblur customers, please use the coupon code provided to you by Heatblur (hint: search your inbox (or spam folder) for the sender “sendowl”, by which these codes were sent by HB).
If you've bought the F-14 module recently, it should be in the same email as the F-14 serial key you've received; if you bought the F-14 module before the VKB Grip was released, it was sent to you in a separate email by HB.
If you have any issues finding your coupon code, please email Heatblur support directly at support@hetablur.se
That said, you might want to post about this in HB's forum here so they are aware that some customers cannot find their VKB discount code... the more folks chime in, the more likely HB will reply in a timely fashion.