two columns of data on separate sheets mark the data that does not exist

ok I will try to explain the best I can, I have two sheets one with lots of data the other is a report I do, on sheet1 column1 I have a list of unit numbers ex 1,2,3,4,5 on unit per row. on sheet 2 column1 I have the unit numbers again but here the numbers repeat with different data accompanying them also sometimes there is a new unit number. I need to somehow compare sheet1 and sheet2 column1 and find the new unit number and either highlight it on sheet2 so I am aware there is a new number or even better copy the new number and insert on sheet1. I have tried conditional formatting not working. sheet1 column1 will only have unique numbers 1 ,2,3,4 etc where sheet2 column2 will have repeats 1,2,2,2,3,1,2,1,3,7 etc I need to that 7. I hope this makes better sense then my last try at help
I am very new at excel

ok I attached a sample file, as you will see on sheet2 12,15,18 are not on sheet1 I need a way to mark 12,15,18 and if possible to add the missing unit number in sheet1 at bottom of list