What can I do right now? I use QGIS version 2.10.1-Pisa. Windows 7 x64bit.
You don't seem to have any layers loaded, that's why it's greyed out.
I've not used this tool myself, but it seems you need to
- select a vector layer,
- set it to edit mode.
Once you've done that, then the plugin's toolbar icon will activate, and you can use it to edit your lines. If you need to change the settings, it's under Vector > Digitize Spline > Settings