How would I properly use a ROBLOX API?

I am trying to utilize this API made by Usering.

This is section of my script.

Content = script.Parent.Content
VisitsText = Content.Page.Visits

PlaceVisitCount = function(plr)
             return tonumber(DecodeJSON(GET("Users/PlaceVisitCount","UserId=" .. tostring(plr))).PlaceVisits)
         end;
VisitsText.Text = "Place Visits: " .. PlaceVisitCount

It should make make the text label the amount of place visits the player you clicked on has.

However this did not work for me.

And I have to use plr to define the player or else it will not work and not display that players stats.

plr is the person they clicked on (I cut that function out of the script because it would clutter up the page.)

How would I use this API correctly?

Usering's API module script

--[[
    DOCUMENTATION

    To use this, make sure to require this ModuleScript in your scripts.

    RbxApi = require(game.ReplicatedStorage.RobloxAPI)  

    CREATED BY: Usering
    CREATE DATE (MM/DD/YY): 04/05/14
    MODIFY DATE (MM/DD/YY): 04/22/14
    NOTES:
        - Make sure HttpService's HttpEnabled is set to true
    VERSION: 1.1
    CHANGELOG:
        - Added api.robloxapi.com/Users/UserId
            | Gets the UserId of a Username
--]]
HttpService = game.HttpService
GET = function(api,params)
    return HttpService:GetAsync("http://api.robloxapi.com/" .. api .. "?" .. params)
end
DecodeJSON = function(str)
    return HttpService:JSONDecode(str)
end

API = {
    Groups = {
        GroupOwner = function(GroupId)
            return DecodeJSON(GET("Groups/GroupOwner","GroupId=" .. tostring(GroupId))).Owner
        end;
        GroupDescription = function(GroupId)
            return DecodeJSON(GET("Groups/GroupDescription","GroupId=" .. tostring(GroupId))).Description
        end;
        GroupName = function(GroupId)
            return DecodeJSON(GET("Groups/GroupName","GroupId=" .. tostring(GroupId))).Name
        end
    };
    Users = {
        FriendCount = function(UserId)
            return tonumber(DecodeJSON(GET("Users/FriendCount","UserId=" .. tostring(UserId))).Friends)
        end;
        ForumPostCount = function(UserId)
            return tonumber(DecodeJSON(GET("Users/ForumPostCount","UserId=" .. tostring(UserId))).ForumPosts)
        end;
        PlaceVisitCount = function(UserId)
            return tonumber(DecodeJSON(GET("Users/PlaceVisitCount","UserId=" .. tostring(UserId))).PlaceVisits)
        end;
        KnockoutCount = function(UserId)
            return tonumber(DecodeJSON(GET("Users/KnockoutCount","UserId=" .. tostring(UserId))).Knockouts)
        end;
        CurrentUsername = function(UserId)
            return DecodeJSON(GET("Users/CurrentUsername","UserId=" .. tostring(UserId))).Username
        end;
        AllUsernames = function(UserId)
            return DecodeJSON(GET("Users/AllUsernames","UserId=" .. tostring(UserId))).Usernames
        end;
        OnlineStatus = function(UserId)
            return DecodeJSON(GET("Users/UserOnlineStatus","UserId=" .. tostring(UserId))).Status
        end;
        UserId = function(Username)
            return DecodeJSON(GET("Users/UserId","Username=" .. tostring(Username))).UserId
        end
    };
    Assets = {
        PlayerOwnsAsset = function(AssetId,UserId)
            owns = GET("Assets/PlayerOwnsAsset","UserId=" .. tostring(UserId) .. "&AssetId=" .. tostring(AssetId))
            if owns == "true" then
                return true
            elseif owns == "false" then
                return false
            end
        end;
        ItemRap = function(AssetId)
            return DecodeJSON(GET("Assets/ItemRAP.php", "AssetId=" .. tostring(AssetId))).AssetId
        end
    }
}

return API