Damn foreignKey member,it was so easy;after a whole day of searching,i found an old post
with an example , which helped me to understand the principle of UiHint working with kendo grid,and it doesn't matter the viewmodel name,the editortemplate name,as long as you point from the main viewmodel to the wanted field to have the wanted template in the back.The foreighKey.cshtml in the Shared\EditorTemplates is only a default template to help you for simple dropdown lists.
In the online web examples there are too many coincidences of naming,same name of viewmodel,same name of cshtml file,same name even in the name of the control,which is not good for understanding the functionality,this naming is only a convention made by microsoft,but as i tested if you point the right viewmodel and view,you have it all togheter.Ex. my main viewModel is ProductViewModel(Datasource of the grid),also i have ProductCategoryViewModel(CategoryId,CategoryName),the EditorTemplate name si CategoryProduct.cshtml,and the name of dropdownlist is Categ,so 3 different names.
My mistake was that i made a property Category of type ProductCategoryViewModel(CategoryID,CategoryName) in the ProductViewModel and to that, i put an uihint attribute,because that way was for inline editing whitout understanding why,
and i should put it from the start to CategoryID from the ProductViewModel like this [UiHint("CategoryProduct")] ,also i had to hide the ProductID,with the attribute [HiddenInput(DisplayValue=false)] even if i didn't use that field in the grid,because if it is in the ProductViewModel then is shown,and you have to put it as i saw for the grid member .Model(p=>p.ProductID).
And now is ok,i have in the popup a dropdownlist for category,of course when loading the grid i have to fill the ViewData object with all the categories.
Also i found a different approach ,without foreignKey,
http://www.telerik.com/community/code-library/aspnet-mvc/grid/dropdownlist-editing-in-grid-s-popup-form.aspx seems more simple,but it's more confusing for me,what method should be more appropiate?!
Thanks anyway to all ,but because there is no comprehensive online documentation,or for clients,a detailed Help file,or something like for the Telerik RAD controls,it's a little difficult to understand every member what it does for each control and how it works especially with MVC.I enjoy this MVC tehnology because of the separation of concerns ,also i like the kendo controls,very useful and good looking,so a good api documentation would help a lot to combine this two tehnologies to build an enterprise web application.Maybe there will be in the future,but also the support team is very ok,and useful until now,so many thanks for your effort.