This is the "client" module you install on your local computer (e.g., your home laptop). It is free and does need activation. Radmin Server (Paid):

Available for companies deploying Radmin across 50 or more machines.

If you are trying to establish a connection between two computers, follow these official steps:

You can also push an activation key from your local Radmin Viewer to a remote Radmin Server:

: Downloading "activation keys" or "keygens" from third-party sites for Radmin is highly risky. Because Radmin provides full remote access to a PC, cracked versions often contain backdoors or malware that allow hackers to take control of your system.

The request for a "kluč aktivácii" (activation key) for Radmin Viewer 3.5.2 typically refers to users looking for a way to bypass the licensing requirements of this professional remote control software.