How does an increase in the price of "non-wage-goods" increase employment?
Wage-goods are goods that a worker with wages might buy, perhaps now more commonly called consumption goods

Non-wage-goods are goods that a rentier receiving profits or interest might buy, including what might now be called capital goods or investment goods

As to how to read Keynes's interpretation of Pigou's position (so actually neither Keynes's nor Pigous's), it might be saying that if non-wage-earners shift their demand towards non-wage-goods, this would have the demand-side effect of pushing up the relative price of non-wage-goods and reduce the relative price of wage-goods, with presumably 


either the direct effect of increasing both the number of wage-earners employed producing non-wage-goods and their wages, 
or the indirect effect of increasing the real wage for all wage-earners (since their wage-goods are now relatively cheaper) above the previous "marginal disutility of labour", and this encourages increased wage-earning employment (what today would be called a substitution effect)