<apex:pageBlockSection title="Product" >
    <apex:pageBlockSectionItem rendered="true">
        <apex:outputLabel value="{!$ObjectType.ZInstallation__c.fields.DiscountProfile__c.Label}" rendered="true" />
        <apex:selectList value="{!ZInstallation__c.DiscountProfile__c}" size="1" rendered="true" >
            <apex:selectOptions value="{!DiscountProfileList}" />
        </apex:selectList>
    </apex:pageBlockSectionItem>
</apex:pageBlockSection>

public List<SelectOption> getDiscountProfileList() {
    List<SelectOption> options = new List<Selectoption>();
    List<ZDiscountProfile__c> profileList = [SELECT Id, Name FROM ZDiscountProfile__c ORDER BY Name];

    options.add(new selectOption('', 'none'));

    for (ZDiscountProfile__c profile : profileList) {
        options.add(new selectOption(String.valueOf(profile.Id), profile.Name));
    }

    return options;
}