*** Update: this issue has been resolved in latest release ***

Currently Signature only supports the public profile url if you have not created a custom username. Signature has been modified to also accept LinkedIn usernames. The updated application is currently under review with Apple and will be available soon.

Currently we only support the non custom url format ie. http://www.linkedin.com/pub/your-name/number/number/number

The workaround until the next update is either
a) Check if you have the old style link stored somewhere e.g. you emailed a friend or business colleague
b) Or create a Custom Hotlink where you can type in the entire LinkedIn Url. i.e. http://www.linkedin.com/in/custom_name (note that it will show the star and not the linkedIn logo)