1. You need to download the All Packages
files that has the same
version as your router from http://www.mikrotik.com/download
2. Extract the user-manager....npk
file from the .zip archive.
3. In Winbox, click the Files menu, and drag+drop the user-manager...npk file into this window. You could also use ftp.
4. After the upload is complete, reboot the router (In Winbox, System > Reboot)
After reboot, I think the logs tell you whether the new package was installed successfully or not... Assuming install was successful, router-ip/userman should be accessible.