Hi TheRealist,
I have modified blog code , you can try it will work now. use this page inline from account page layout , follow step as metiond  blog. please let me if this works for you and mark the best answer.
Apex Code:

public class OpportunitySearchController {

    //added an instance varaible for the standard controller
    private ApexPages.StandardController controller {get; set;}
    // the actual account
    private Account a;
    // the results from the search. do not init the results or a blank rows show up initially on page load
    public List<opportunity> searchResults {get;set;}

    // the text in the search box
    public string searchText {
        get {
            if (searchText == null) searchText = 'Acme'; // prefill the serach box for ease of use
            return searchText;
        }
        set;
    }

    public OpportunitySearchController(ApexPages.StandardController controller) {

        //initialize the stanrdard controller
        this.controller = controller;
        this.a = (Account)controller.getRecord();

    }

    // fired when the search button is clicked
    public PageReference search() {
        if (searchResults == null) {
            searchResults = new List<opportunity>(); // init the list if it is null
        } else {
            searchResults.clear(); // clear out the current results if they exist
        }
        // Note: you could have achieved the same results as above by just using:
        // searchResults = new List<categoryWrapper>();

        // use some dynamic soql to find the related opportunities by name
        String qry = 'Select o.Id, o.Name, o.StageName, o.CloseDate, o.Amount from Opportunity o Where AccountId = \''+a.id+'\' And o.Name LIKE \'%'+searchText+'%\' Order By o.Name';
        searchResults = Database.query(qry);
        return null;
    }

}
VF Page Code

<apex:page standardController="Account" extensions="OpportunitySearchController"> <style type="text/css"> body {background: #F3F3EC; padding-top: 15px} </style> <apex:form > <apex:pageBlock title="Search for Opportunities by Keyword" id="block" mode="edit"> <apex:pageMessages /> <apex:pageBlockSection > <apex:pageBlockSectionItem > <apex:outputLabel for="searchText">Keyword</apex:outputLabel> <apex:panelGroup > <apex:inputText id="searchText" value="{!searchText}"/> <apex:commandButton value="Search" action="{!search}" rerender="resultsBlock" status="status"/> </apex:panelGroup> </apex:pageBlockSectionItem> </apex:pageBlockSection> <apex:actionStatus id="status" startText="Searching... please wait..."/> <apex:pageBlockSection id="resultsBlock" columns="1"> <apex:pageBlockTable value="{!searchResults}" var="o" rendered="{!NOT(ISNULL(searchResults))}"> <apex:column headerValue="Name"> <apex:outputLink value="/{!o.Id}">{!o.Name}</apex:outputLink> </apex:column> <apex:column value="{!o.StageName}"/> <apex:column value="{!o.Amount}"/> <apex:column value="{!o.CloseDate}"/> </apex:pageBlockTable> </apex:pageBlockSection> </apex:pageBlock> </apex:form> </apex:page>

Best Regards,
-Vivek