Create a column based on a lookup in another table

Hi, I've spent the last hour trying to understand this and google around. I must be missing something completely because it simply cannot be that difficult. Alright, I have the below two tables:

___________________

Table 1: 

AccNumber |

Table 2:

AccNumber | AccDescrip

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All I want to do is create an additional calculated column called "Main" in Table 1. Where for each row, the equivalent vlookup function would be: 

Main = vlookup( Table1.AccNumber * 1000,  Table2,  2,  False)

Could anyone please shed some light as to what my formula would be for creating a calculated column?