How long ago did you buy Pro?
If you bought Pro from the official site Lime Wire » Official Website & Free Download, BitTorrent « (payment to LimeWire LLC shows on your CC statement) & your Pro benefits are still current, then you can update to LW 5 Pro without paying. If your Pro benefits have expired then your email address won't be in LW's database any more & so it won't be recognised.

If you bought Pro with 6 months of free updates & email support then you MUST still be within that 6 month timeframe (ie you must have paid for Pro less than 6 months ago).
If you bought Extended Pro with 12 months of free updates & email support then it needs to be less than 12 months since you purchased.

After you buy Pro, you'll recieve an email from LW with a link to your personal download page. Have a look & see whether you have that email saved. If you do, just use the link provided

If you bought Pro from a scam site then your email address would have never been in the official database & so it won't be recognised if you try to download Pro from the official site..

If you're sure that you paid Limewire LLC for your Pro & your benefits are still current, try every email address that you've used around the time of buying Pro: Lime Wire » LimeWire PRO Lookup

And if you still have no luck, send a PM to limenathan here & provide him with any relevant info: limenathan (click on link)