you should check first that the item you want to use is available by right clicking on one of the xyz views, select create model, try to find your item, if not found click on cancel and use reload models in file tab and look for it again, if not found then the item was not saved in the models directory, move it there, or if directory don't exist in your darkmod install, create directory models in the same directory as where all the pk4 files are. if item found then you can do the following.
so i dont screw anything up I always move what am making a collision box for to the editors xyz origin and place the base of the item on the xyz origin so I can move the collision box origin onto that origin.
create a box around your object textured with the collision texture, or select the collision texture and use that to make a box around the item, convert collision box to a func_static, press v to move the origin of the collision textured box to the bottom of the box(this is where it would rest if dropped on the ground), press esc twice so nothing else is selected, select collision box, save selected as a collision model, select the model you want to add the collision box to in the model selection screen, click on ok to select it or cancel to not select it.
to un-taff what you done find the collision box you made (<what ever its named>.cm) and delete it. will be in the model directory with the same name as the model you selected if you did not select one it might have been save as cm
to make a useable item there are three types. to use place the one you want in your world, eg atdm:moveable_custom_item
change name to something like what it is - name : bunch of grapes
change inv_icon path to a tga image or dds image - inv_icon : guis/assets/hud/inventory_icons/bunchofgrapes
change model path to path of your model with the collision box ase/lwo - model models/darkmod/bunchofgrapes.ase
if you can't find your model, just use the reload models option in file
the collision box is auto loaded with the model
if its the one that doesn't go in your inventory then you dont need to set the inv_icon path
it still needs a unique name so objectives and scripts can access the item when they need to