If you are an existing player and already have the client then make sure you're patched with the 1.3 Client. There's a Patch Stone by the Britain Bank. Download the patch and extract it to your Legends of Sosaria game folder.
Note: If you downloaded the client after the patch date then your client is already patched and you should be good to go. The town criers will let you know when there is a new patch.
If you are a new player, then welcome! You'll need our custom client to enjoy what we have to offer. Attempting to play on a different client will cause you to not be able to see many of the items in game and several monsters will be invisible (but still be able to attack you). It's in your best interest to keep your client patched correctly.