The easiest way to understand Studio textures is to look at some examples in the Surfaces tab and see what gets assigned where.
The basic idea is that texture maps are assigned to the diffuse channel, so load your item into Studio, select it, go to the surfaces tab and you should see it listed on the left hand side of the pane. If it is grouped into multiple groups, click on the triangle to the left of its name to display the full list of its groups. Click on the triangle at the left of the group you want and you will see a list of all the active channels, click on diffuse. This will bring up ‘Diffuse Color’ and ‘Diffuse Strength’ on the right. Click on the little box to the left of ‘Diffuse Colour’, you should see a list of all textures currently loaded into Studio, but yours isn’t yet, so at the very start of the list, click ‘browse’ which brings up a standard file-finder window, find your file, select it, and you’ve loaded your texture.
You now need to adjust it to look reasonable in Studio, so may need to adjust some of the other channels - I’d suggest loading some other stone objects from your runtime to look at how they’re set up and shamelessly copying settings Adamr001’s tutorial on converting from Poser textures may also give you some pointers:- http://forumarchive.daz3d.com/viewtopic.php?t=156773
Having loaded the texture you now need to save it. Select the relevent groups in the Surfaces Pane, then press file/save as/shader preset and follow the pop-ups through taking the default options.