I don't have this add on but I can tell you how to do it.
Click on the GB App on your iPhone. At the Glucose Buddy Main Menu take your finger and slide the screen to the left. Here you will see a lone Premium Features Icon. Click on the icon and it will take you to a screen that says Purchase BP & WT add-on >. Click on that and it will ask you to Confirm Your In App Purchase. Choose BUY.
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Regards;
Danny 
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