Sorry, too busy reading message body, didn't fully read title. Since I no longer have access to NX6, I'll try to explain this without examples.
There are two ways you can go about using this deformable part in an assembly. You can either drive the assembly with the spring length or have the assembly drive the spring length.
In the first case, start by adding the spring to the assembly and mating it to one of its mating parts in the assembly. Once you finish mating the spring you will be prompted for a length value for the spring, enter the desired value. Then add/mate the part that is connected to the other end of the spring. You will see an entry in the part navigator for the deformable part, change this value and the length of the spring will change, driving the overall assembly.
For the second case, create the assembly using all the parts except the spring. Then use the measure distance function to create an associative distance measurement that reflects the distance between the spring mounting faces. Then add the spring to the assembly and mate it to one of the mating faces plus what ever axial constraint is needed. When you are prompted for the spring length, use the previously created measurement as the value for the spring length. The spring should fit in the space perfectly. When you cycle the assembly, the measurement will update and then the spring length will update also. Note that the update will not happen in real time if you are dragging the assembly components to cycle the assembly, it will update once you click "apply".