For using method from custom library what you need is adding namespace in views/web.config
for example if you want to use linq methods you need to put something like this
<add namespace="System.Linq" />
When you open the web.config file you already see some namespace there. After add namespace you can use that method in any of your views.
if this still not work in VS 15 , Close the VS and Restart it. it should work.