I'm a little surprised no one has chimed in on this one with more info about the EXS24 samples. Have you loaded the Orchestral Jam Pack that came with Logic Studio? If so, you should have a number of EXS instruments with solo strings. I don't use them, so I don't know how good they are, but I just looked them up and found quite a few.
Open an EXS24 instrument, and then go to the Library tab, or click on the little instrument selection window in the EXS24 player itself (it's above the cutoff knob under the Cut/Paste functions). From there, select Factory/09 Orchestra/09 Strings, and you should see a number of presets. In my folder I have Sampler instruments for Basses, Cellos, Full Strings, Violas, and Violins. Included on each of those instruments are presets for Legato, Pizzicato, Staccato, Tremolo and Trills.
If you don't see that folder, or don't see those presets, leave Logci, open a Finder window and follow this path: HD/Library/Application Support/Logic/Sampler Instruments/09 Orchestral/09 Strings. You should have about 40 presets in this folder, the same ones that should show up in Logic by doing the steps mentioned above.
If you don't have that folder on your HD or don't see presets there, then you're missing content. I'd suggest going into Logic's installer and reinstalling the Orchestral Jam Pack.
Hope this helps. Good Luck!