The solenoid module goes into slot 8 on the cRIO by default. If you have a second solenoid module, put that into slot 7. Every solenoid connected to one module has to operate on the same voltage e.g. all 12V or all 24V.
Code:
Solenoid *m_solenoid_1;
...
m_solenoid_1 = new Solenoid(1);
That code assumes the solenoid module is in slot 8. You only need to two argument version if you are also using slot 7.