I have two tables: one in Excel and one in .txt format. I need to move these into ArcMap 10.1 and combine them with a polygon shapefile of counties in a state. However, I don't know how to do this.

Afterwards I need to calculate the mean center of the population of the state by using the data provided by the two tables. Each table consists of a list of the counties and the population of each county for different years. The fields are the years and the first field states the county names. 

I've tried merging the two tables in Excel and then importing my table in ArcMap, then when I try to do a join between the shapefile and the table I get null values where the attached table values should be.

Can someone explain to be how I would exactly get this data joined to the polygon shapefile?