Hi Susanna,
First you have to set up your device.
Set the mode-switch to "Repeater"
Setup the WiFi-device on your computer. Find the "dlink-<4 characters>" and provide the password that is provided with the papers of the D-Link
Next open the browser-page of the D-Link with "http://dlinkrouter<4-characters>/index", no password needed.
Setup you WiFi on the D-Link. Use the manual mode. Search your accesspoint and provide your password.
When this is finished your computer should have Internet-access.
Good luck!