Consume Extension method in non static class

All of the examples for extension methods that I have seen consume the extension method in a class like:

class Program
{
    static void Main()
    {
        ...Call extension method here
    }
}

These seem to work because the consuming class is static. Is there a way to consume an extension method in a non static class like below? I can't seem to find any examples like this.

I have my Extension Method class:

using System;
using System.Collections.Generic;

namespace AwesomeApp
{
    public static class LinqExtensionMethods
    {
        public static IEnumerable<T> FindItemsBeforeAndAfter<T>(this IEnumerable<T> items, Predicate<T> matchFilling)
        {
            if (items == null)
                throw new ArgumentNullException("items");
            if (matchFilling == null)
                throw new ArgumentNullException("matchFilling");
            return items;
        }
    }
}

And I have my class that consumes the extention method

namespace AwesomeApp
{
    public class Leaders : ILeaders
    {
        var leaders = GetAllLeaders();

        var orderedleaders = leaders.OrderByDescending(o => o.PointsEarned);

        var test = orderedleaders.FindItemsBeforeAndAfter(w => w.UserId == 1);
    }
}

If I call the extension method from a static class I do not the the 'Extension method must be defined in a non-generic static class' error:

public class test
{
    public void testfunc()
    {
        List<int> testlist = new List<int>() {1,2,3,4,5,6,7,8,9};

        testlist.FindItemsBeforeAndAfter<int>(e => e == 5);

    }
}

I have read through all the stackoverflow answers I can find on the non-generic static class error and they deal with writing your extension method but don't deal with consuming it.

So the question is: If using an extension method with a non-static class is not possible then is there any way to do something that works in a similar way? for example it can be called as .ExtensionMethod not Helper.ExtensionMethod(passedObject)??

Resolution: I thought I cut and pasted the extension method from public class Leaders : ILeaders to its own class so that I could make it static but I actually just copied it. The compiler error was pointing to the class name so I did not see the extension method still at the bottom of the file. The error message is accurate and everyone that answered is correct.