You're using Excel 2010. You need to create a Pivot Table and you're looking for the count distinct function in Value options section only to find it's not there. So what do you do? Well if you're using Excel 2013 you can do it the easy way. However if you are using Excel 2010 or earlier, then you need a little magic formulas.
This video will show you how to use the SUM, IF, FREQUENCY and MATCH functions to count distinct values. You can then use these values to create a distinct count on a Pivot Table. This should tide you over until you upgrade to 2013 or get onto using Power Pivot or Power BI or the like.