Jae
By dialog box I just mean the window that pops up with the violin, cello and bass.
As Kalle said, it's hard for us to help when you can't tell us that your using a VST.
Don't take it the wrong way, we are here to help and want to help.
You have to do some things on your own first. Do like Mettelus suggests and just go through all the free tutorials that come with X3. Then look to Cakewalk TV on youtube to find even more instructions.
Also, we need to know how your going to input info on the part you want to create. Do you have a midi controller? If not, the easiest thing to get going is to look at the top right of the String VST interface, and the second icon from the left, is the Step Sequencer I told you about, click that button and it will bring up another interface that you can use to trigger the String VST.
That should at least get you going.
HTH