You can dose your model by adding a dose object (e.g. a Repeat Dose) to the species you want to dose. In the image below, the species is called 'Drug'.
Note that in this case the 'TargetName' property of you dose is the species 'Drug'.
If you want to simulate an infusion, you can set the 'Rate' property to the infusion rate you wish to achieve. The dose object will infuse the value you give for the 'Amount' property of the dose at the value you give for the 'Rate' property, where infusion time = amount/rate.