excel - How to find if value exists in column, and enter if it not found? -

i'm working table , user-defined variable may or may not exist in column a. if exist, need able select cell. if doesn't exist, need enter user-defined variable cell. think if/then/else statement should work, can't figure out how syntax work.

lance on right path, think made small error. if understand correctly, following need:

if rngcell.value = uservariable   rngcell.select else   rngcell.value = uservariable end if