Impossible to attach splines

Hello,

I'm just new to 3D studio, so exuse me for the simplicity of my question.

I did a shape with splines, and now I just want to get all those splines connected to be only one, so I can extrude properly my shape.
I tried select one and to do Geometry>attach but nothing happen. When I click on Attach mult. nothing appears in the list.
I must have done something wrong when I created my splines.

Does someone know ?

thanks!

Ok, thanks for your answer.
I've selected my vertex and welded them one by one.
It did stick some of my splines together. Now I ended up with only 3 splines that refuse to stick together.

Here is a picture to show my situation that I believed to be simple.
my 3 splines to be stuck are the outer circle and the shapes on its right and on its left.
How would you join those 3 shapes ?
thanks