Change and add fields in Force.com IDE
If you are using the latest Force.com IDE to develop s-controls or Apex code, you know how nice the latest upgrades are — for editing code, testing, and staying in sych with Salesforce.
I’ve just discovered something else really great — you can edit metadata for objects and fields in your Salesforce instance. (It has to be a sandbox or developer instance, but you can then package and "deploy" your changes to a production instance.)
Have a custom object or a picklist in one instance and want to copy it to another? Need to change a bunch of field names or picklist values? You no longer need to package the items and install them from the appexchange, or go through all the pointing and clicking. Instead, you make the changes in the object metadata XML files in Eclipse — you can use the XML design tools, or just edit the raw XML:
At first, I was getting a lot of errors when saving objects. There is a bug, and I found the workaround here. Basically, there is this file called package.xml — if you are adding an entirely new object (via the New menu), the Force.com IDE updates that file with your additions. But if you just add custom fields, or if you copy and paste anything from one instance to another, it won’t be in package.xml, so you’ll get an error when saving or synchronizing to Salesforce.
The trick is to open up package.xml, find or create the section that lists the type of thing you want to create, add the tags, save package.xml, and THEN save your new items. For example, putting this code in package.xml lets me add the three custom fields shown in the Lead.object file above:
Lead.Address_Type__c
Lead.Birthdate__c
Lead.Communication_Preference__c
CustomField
Be sure to add the code to package.xml before EACH thing you add/paste and save, because Salesforce synchronizes package.xml when you save, taking out any entries that aren’t actually in your instance.

Hi,
I was trying to add new custome field called comment1 in object through force.com editior.This object is in package.
First I change package.xml and write
ObejctName__c.comments1__c
saved.
Next I change Admin.profile and wrote
true
objectName.comments__c
Next changed objectName.xml
comments1__c
Comments1
32000
LongTextArea
10
When trying to save this file I am getting error:cannot add new filed to installed packege.
Any guess,solution?