Hi Heli,
Have you tried to install SAPUI5 in Eclipse IDE or in Hana Studio? In my case I only downloaded and installed SAP Hana Studio and then installed SAPUI5 in Hana Studio.
To install SAPUI5 in Hana Studio:
Go to Help->Install New Software
In Avaliable Software screen add below two entries:
Then in the Work With, select SAP Development Tools for Eclipse Juno and select UI Development Kit for HTML5.
If you tried installing SAPUI5 in Eclipse IDE, which Eclipse IDE are you using? is it Eclipse Juno?