How can I add an injured reserve (IR) slot?

Go to League » Settings » Edit Roster Requirements » Custom » Add Injured Reserve.

After you add the IR slot, go to League » Settings » Edit Roster Requirements » Custom » Edit (across from the IR icon) and set the desired minimum value. Only players that meet this level can be placed on your fantasy team's IR. If a player on your team's IR becomes healthy and is no longer eligible for IR, your team will not be allowed to add free agents until the ineligible player is "activated" from IR (i.e., assigned to the active roster—either benched or starting).