Multi Roblox is a software that grants you permission to run multiple Roblox instances/games on different Roblox accounts at the same time.
Any software may trigger a reaction with your anti-virus, this does not mean it actually is a virus. These false positives are usually caused by Obfuscation/Packing and Encryption.
If you are unable to download the software ensure your anti-virus is disabled, this will prevent the deletion of the files. If your brwoser states that the download was "blocked" simply head over to the downloads section on your browser and press "keep".
Ensure you have the recommended visual C++ redistributables for Visual Studio, you may download the x64 and x86 version here, once installed restart your device.
Ensure you have the latest version of .NET Framework, you may download it from here, you must download the runtime and install it.
This could mean many things, certain Software require weekly updating for the most recent versioning of the game, if the execution of a script is crashing your game this could mean the Software does not support the script. You can check if the Software is up to date by checking the date on the website.