This answer sounds stupid? It is!
But I'm stuck in front of my rim, with that stupid valve that won't get in its hole!! There has to be a trick to get it in!!
Someone tell me...I'm feeling soooo dumb!!!

 

Try hand soap with a touch of H2O. If that don't work as a last resort use WD40.

 

assuming the valve stem is a standard type valve stem it will have to be inserted from INSIDE the rim. YOu and push and squeeze all you want you wont force it thru the hole from the outside. SOme large truck tire style valve stems even have a locking nut on the inside of the rim but i very much doubt atv sized stems have that. You will have to pop the bead on the tire.. and reach in from the inside to force the stem thru the rim hole. Soapy water is a good choice to 'lube' the stem so it pops in easier

 

As a personal habit, once the soapy water coated valve stem is seated in the hole, I rotate the valve stem 1 or 2 times to assist in sealing the stem to the rim. Occasionally you will get a piece of trash between the rim and stem that will prevent the stem from sealing and the rotation of the stem generally allows it to seal. One of those inexpensive "T looking" valve stem tools works good for giving leverage to pull the valve stem through the hole and rotate it. It the stem is too much to pull through by hand, you can always use a pair of Channel lock pliers to grip the tool under the "T" and pry upwards against the rim for more leverage.