I do not buy spring loaded shower curtain rod too often so I often forget how to install and adjust one. After the move I bought a IKEA Savern show curtain rod and for a few minutes cannot figure out how to adjust the length. Then it came back to me. You just PULL. There is a lot of built in resistance inside the rods to keep them attached. You just have to pull really hard to extend the rod until it is just a little longer than what you need.
Another tip: If you ended up pulling the two rods apart, make sure you insert the smaller rod back into the larger rod spring side last. The spring at the end of the smaller rod is supplying the tensions.