What's the exact version you are using? Version 3.8.1 does not exist, the latest is 3.2.3.
It's also important to understand that if you are trying to control traffic of another computer (e.g. your brother's), it will not work unless the software is installed on or before the gateway as described here.
In such case, all you need to do is to create a rule like this:
Rate: 50 KB/s
Also check the built-in event log to see if there are any error messages or whether the trial version has expired. If everything is okay, the above rule should limit everything in and out of the computer by 50 KB/s.
You can check the usage report to see if the rule captures traffic.