Hi,
Seeing you havnt had many/any advice,i will try to help.
It is hard to put weight on a dog by JUST feeding kibble,so you must add fresh foods along with dry if thats what you feed.
Good foods for putting weight on are:suit(from butcher) beef dripping,fatty cuts of meat,marrow bones,chicken wings,lots of raw meaty bones,lamb flaps are excellent,corned beef/briskett,whole weat pasta,brown rice,my dogs go nuts for pork crackling.
Make sure to feed plenty of fresh meats and veggies.
One of my dogs was always on the slender side,i too asked for advice but got the same replies as you did
There is also a recipe floating around on the net called "satin balls" this mixture can put weight on a dog in a matter of days

alot of show folk feed this before a show to beef up their dogs,unfortunatly if you have a slim,fit dog these days it is often overlooked for the more chunkier types :(
Type "satin balls recipe" into your search engine and the recipe will come up.My dog also didnt have a very good appetite,i tried plying her with treats and filling up her food bowl but she only ate what she felt like and left the rest,but by following the above she eventually put on weight,she is now a healthy solid,muscular dog :D Be patient tho,dont try and put weight on her too quickly or else you could do damage to her joints.
If you need any more tips/advice please ask,i have researched this putting on weight thing till my head exploded!
good luck
christine