[SalesForce] Custom Button and Link can’t use VF Page

You can call your method on button click as shown below

<apex:page standardController="Opportunity" showHeader="false" sidebar="false">
<script>
{!REQUIRESCRIPT("/soap/ajax/29.0/connection.js")}

{!REQUIRESCRIPT("/soap/ajax/29.0/connection.js")}
//This is your javascript
function SaveNote()
{
    var note = document.getElementById('frmid:pgblk:pgblksection:descriptionfield').value;
    if (note != null) {
    var project = new sforce.SObject("Opportunity");
    project.id = "{!Opportunity.Id}";

    var date = "{!TODAY()}";
    // var user = "{!$User.Alias}";
    var user = "{!$User.CommunityNickname}";
    var oldnote = "{!JSENCODE(Opportunity.Description)}";
    var prepend = date+" "+user+" - "+note+"\n";
    var newnote = prepend+oldnote;

    project.Description = newnote;

    var result = sforce.connection.update([project]);
    window.location.reload();
    return false;
    }

}

</script>
<apex:form id="frmid"> 
    <apex:pageBlock id="pgblk">
        <apex:pageBlockSection columns="3" id="pgblksection">
            <apex:inputfield id="descriptionfield" value = "{! Opportunity.Description}" />
            <apex:commandButton onclick="return SaveNote();" value="Add Note" /> 
        </apex:pageBlockSection>
    </apex:pageBlock>
</apex:form>  

You need to use recordSetVar on your page to make it available for custom List view button.

<apex:page standardController="Opportunity" recordSetVar="opportunities" tabStyle="Opportunity" extensions="tenPageSizeExt">
    <apex:form >
        <apex:pageBlock  title="Edit Stage and Close Date" mode="edit">
            <apex:pageMessages />
            <apex:pageBlockButtons location="top">
                <apex:commandButton value="Save" action="{!save}"/>
                <apex:commandButton value="Cancel" action="{!cancel}"/>
            </apex:pageBlockButtons>
            <apex:pageBlockTable value="{!selected}" var="opp">
                <apex:column value="{!opp.name}"/>
                <apex:column headerValue="Stage">
                    <apex:inputField value="{!opp.stageName}"/>
                </apex:column>
                <apex:column headerValue="Close Date">
                    <apex:inputField value="{!opp.closeDate}"/>
                </apex:column>
            </apex:pageBlockTable>      
        </apex:pageBlock>
    </apex:form>
    </apex:page>

Adding Custom List Buttons using Standard List Controllers