I have downloaded a .stp file of MAX14878 foot print. I tried to add it to the library through manage footprints options. It shows a table where it is added. But I am not able to find it during edit symbol fields for the project. Can anyone help me in adding an external footprint to kicad. I use version 5.1.4
".stp" is the file extension for the 3d CAD exchange format called "step": https://en.wikipedia.org/wiki/ISO_10303-21
KiCad footprints have the file extension ".kicad_mod".
There is not really a direct way to convert a step file into a footprint as it does not normally contain the info you would need (Step files typically represent the component not the interface to the PCB).
Your component comes in either an 8 pin or 16 pin SOIC package (wide variation). There might already be a fitting footprint in the Package_SO lib. If not then you can easily generate footprints for such parts by use of our IPC footprint generators found here (see readme.md for docu) https://github.com/pointhi/kicad-footprint-generator/tree/master/scripts/Packages/Package_Gullwing__QFP_SOIC_SO
And of course you can also directly make such a footprint directly in KiCads footprint editor https://forum.kicad.info/t/tutorial-how-to-make-a-footprint-from-scratch/11092/
Tip for the future: If you have a problem with getting some asset into any program then link to the source of your asset. For KiCad library assets the least you should provide is a link to the datasheet of your component. If the drawings of the package are separate from the datasheet then link to them as well.