Check out OpenTK.Input.

From there you can get access to OpenTK.Input.Mouse and

Some examples from the page include:

To check whether a button is pressed:

using OpenTK.Input;

var mouse = Mouse.GetState();
if (mouse[MouseButton.Left])
{
    // Left mouse button is pressed
}

To check whether a mouse button is not pressed:

using OpenTK.Input;`
 
var mouse = Mouse.GetState();
if (!mouse[MouseButton.Left])
{
    // Left mouse button is not pressed
}

Check out OpenTK.Input.

From there you can get access to OpenTK.Input.Mouse and

Some examples from the page include:

To check whether a button is pressed:

using OpenTK.Input;

var mouse = Mouse.GetState();
if (mouse[MouseButton.Left])
{
    // Left mouse button is pressed
}

To check whether a mouse button is not pressed:

using OpenTK.Input;`
 
var mouse = Mouse.GetState();
if (!mouse[MouseButton.Left])
{
    // Left mouse button is not pressed
}