If you haven't found a flat or room yet, you can either apply directly at the Wohnungsbaugenossenschaft Ilmenau (WBG) or the Ilmenauer Wohnungs- und Gebäudegesellschaft (IWG) or on the relevant platforms, e.g. wg-gesucht.de or </ignore>immowelt.de. For the transitional period, there is an Jugendherberge and the Studierendenwerk offers rooms for Kurzzeitvermietung offered.
When you move to or within Ilmenau, you must register within two weeks at the Residents' Registration Office (Einwohnermeldeamt). A slight delay is not usually a problem. If you do not re-register, in the worst case you will have to pay an administrative fine.