I have not come across an "add-in" for flowcharting.
You did not say which version of Excel is being used but Excel 2010 (and probably 2007 - cannot remember) has flowchart elements (e.g. shapes and lines) on Insert/Shapes.
If I want to draw flowcharts myself, I use
RFFlow (
http://www.rff.com/index.htm) that I find is easier to use than SmartDraw (
http://www.smartdraw.com/specials/ppc/smartdraw.htm?id=343825&gclid=CP-os8rj0K0CFRIhtAod4T4t2A).
To flowchart VBA code I have found two applications.
1. Code Visual to Flowchart (
http://www.fatesoft.com/)
2. Visustin v6 Flow chart generator (
http://www.aivosto.com/visustin.html)
Code Visual to Flowchart uses what I call 'standard' flowchart connecting lines but there is a limitation with printing -you have to export the flowchart to Word, PowerPoint, Excel etc. but it can only be printed on ONE page unless you do it via Excel. However, Excel seems to have a bug in that the 'boxes' lose the last line when you come to print - I have written some VBA to change the size before printing to overcome this.
But support for the software appears to be non-existent - according to the site there should be some but I have tried to contact them (both Support and Sales) at least 4 times and received no reply.
Visustin v6 costs more but I think is better if printing is required - printing and viewing as PDF is best and seems to be recommended. The connector lines are what I would call 'non-standard' (but I have been using flow-charts since the late-1970s).
For printing VBA flowcharts it is advisable to do it with small 'sections' of code at a time.