Dylon is a great dye! I used it to dye organic bamboo velour.
Are you using 100% natural cloth? If not, the dye won't catch.
If so, then you are good, and yes you need the salt for the dye to catch. I remember reading on the package and the website on the step by step instructions. I dyed the fabric a special way though, it's called LWI. I like the effect. The dye hasn't faded or washed out.
Now, if you have unnatural fibers or blends, the dye is less likely to catch and you need dye for that. idye poly is for polyester (which is basically plastic). If you have a blend (like cotton poly) then you'll need idye natural too.
I hope that helps, good luck!