It states you have it linked on the forums here, make sure you've rejoined the server after your purchase and that you're using the correct steam account.
Hi longmajs,
We have just taken a look into this for you and can see that you've linked your Steam profile to the following account: steamcommunity.com/profiles/76561198245848618 Is this the same Steam account you use to play on the ZARP servers? Please let us know if you use a different profile and we will update this for you.
Thank you for supporting ZARP!
Hi longmajs,
We have unlinked your account now. Click on the ZARP VIP tab at the top of the site and follow the instructions to Sign in with Steam with the same Steam profile you use to play on the ZARP Garry's Mod servers. You will need to re-connect to the server after doing this to see your upgrades in-game. Thank you!