Err,
don't you know how to establish an EXS24 plugin into a software instrument channelstrip?
Look at your software instrument channelstrip (left side of Logic window). You should see the fader, and above it some rectangles with an "I/O" above them. Select the first rectangle beneath the "I/O". Click *and hold* your left mouse button to open up the list of available software instruments. Select the EXS24 sampler.
In the plugin window which opens now, there's a silver region with some knobs. Just above "cutoff" theres a display field. Click into this display. Select factory -> pop strings -> violin. Or, if you have the Jam pack installed, factory -> orchestral -> strings -> violin.
Or look under "Garageband" for the default strings. It's the same as "pop strings".
If you don't want to use the EXS24, open one of the Garageband software instruments. They are basically the same as the EXS24 instruments but with another plugin window with less options to fiddle with.
And a strong request: As these are most basic things, so read the manual. Logic isn't Garageband and has a steep learning curve. You will need the manual to learn it.
Fox