use a lubricant of any kind.
As long as you're sure you have the correct module, then a little force is sometimes required.
The module is keyed so that it can only go in one way. Put the retaining clips in a half-way position or closed just enough that the module will still squeeze in. That will cut down on the force you need by not fighting them so much. Memory modules on a new build is something I always install out of the case on a flat surface. Some cases don't have much support for the mainboard underneath.