Are you tracking the inventory on those other products? Tangible goods that do not have an inventory value to change in E-junkie won't show up in the window to select products for the package.
But that's okay, because if you are not tracking the inventory you don't need to do anything special in our system. If you want to sell Product A and Product B together without tracking inventory, simply create a Product C and ship both A and B together for any buyer that selects C.
Do the products you want to bundle together have variants/variation options for different sizes and colors, etc? A packaged product won't list those options with its button so you may wish to not to combine those products together in a single package.