Registration for most courses within the Chemistry master's programme is done via the UvA, even if the courses themselves take place at the VU.
To be able to access the UvA systems, an UVaNetID is required. All students of the Master Chemistry (joint degree) programme recieve both an UvANetID and VUNetID. Please note though that you need to fully finish your master's admission process (having shown your bachelor's diploma and paid tuition) before both your VUNetID and UvANetID are activated. Up till then, only the NetID of the university where you originally registered is active.
Course registration for the first semester is done via DataNose. Select the courses which you would like to enroll for and press submit. General course information can be found in the online UvA course catalogue.