Go to Settings->General->Restrictions
To enable in-app purchases you need to first enable restrictions and enter a 4-digit code.
So, do this now. Click "Enable Restrictions"
If you have never entered a 4-=digit code before then make one up, but do remember it.
Once you have enabled restrictions you can then slide the "in-app purchases" over to show green (slide the slider to the right), thus enabling it.
Once you have done this, go back up to the top of the screen and click "disable restrictions" to set things back how they were at the start.
Job done !