To assign slaves for i.e ddos missions (not for RCE Source missions) you need to upload and install the program you wish to assign to a slave and if you do not have any other viruses that need to be installed

(vddos, miner, spamware, and trojan)

will be assigned automatically but if you do have a different virus installed on a player/npc you have to open the Slaves tab

(Click on the bottom left paw then use the arrow until you see "Slaves" then click that to open it)

you can scroll to the IP you want to assign the virus to if it is highlighted in blue if it is active but if it's highlighted orange then it is not active, if a virus is already active you have to click "None" then click the desired virus (Note: you cannot have two active installation viruses on anyone at any given time) after that, the desired virus should be installed!