To access the tileset import options, start by adding a completely empty sprite (I'm not doing pictures right now as I have a killer migraine...), now go to the animator tab which is typically on the right hand side of the work area somewhere. Click on the empty frame with the number '1' underneath it and click on 'import'
You are then given a file browser to choose what you want to import. You can use this to either import many images (for example, a sequence of images), or you can use it to import a tileset image (or both if your heart desires)
Once you have selected the image(s) you want, press enter/ok and this fancy little import window will appear (give it some time, with many images/large images it has to load)
On this window are all the options you need to split an image into tiles and stuff. You'll have to play around with it yourself as I haven't done it in months and like I said earlier, I have a whopper migraine...
I hope that was of some help