How long ago did you buy Pro? Are you sure that you bought from the official site (payment to LimeWireLLC will show on your credit card)?

LW don't issue you with a username or password to download Pro - after payment, you recieve an email from LW with a link to your personal download page. Have a look in your saved emails & see whether you can find that.

That link's valid for as long as your Pro benefits are current (ie if you bought Pro with 6 months of free updates & email support then the download link works for 6 months, likewise if you bought Pro with 12 months of benefits the link works for a year). Email addresses only stay in LW's database while the user's Pro benefits are current.