I recommend to install plugins via your WordPress Dashboard.
For manual installation:
Download mklasen's Thumbnail Slider Plugin (
http://wordpress.org/extend/plugins/mklasens-thumbnail-slider).
To install mklasen's Thumbnail Slider follow these steps:
- Upload the folder "mklasens-thumbnail-slider" to your WordPress plugin folder (/wp-content/plugins/)
- Activate the plugin through the 'Plugins' menu in WordPress