You'll have to load the whole table into memory and then search column a for an entry that matches the user input. Then you use the index of a to get the value for b.
To load the table into memory, use the MATLAB function xlsread.
data = xlsread('myFile.xls')
There are options to restrict how xlsread opens the file if you need to read specific ranges of cells or cells that include text. Look up the documentation if necessary.
The above function call is assumed to work and the file contains two columns "a" and "b".
Finding your "a" value would be something like this:
and saving your "b" value would work like this:
In this case, if "a" exists more than once in the data, you'll get an array of "b"s associated with those "a"s. You can combine the above lines like this:
b = data(data(:,1) == a,2);