You will only be able to modify component content via Experience Manager (XPM), if you add them in your Dreamweaver template in a specific way. If you just add the field like this :
the value of your field will be inserted in the page, but you won't be able to edit the field via XPM.
If you output a field like this, Tridion will output the necessary XPM code for you:
As from Tridion 2013 the previous syntax is deprecated and the following should be used:
These XPM tags are needed, so XPM knows which piece of content corresponds to which field in your component.
More examples can be found in the Tridion documentation : http://sdllivecontent.sdl.com/LiveContent/content/en-US/SDL%20Tridion%20full%20documentation-v1/GUID-99143B2A-5B69-4EED-B470-DD70559AE616
I am not sure if you are using regular Tridion templating, or DD4T, but if you wish to add XPM in your DD4T implementation, there are some helper functions in the dd4t framework to support XPM as well. You can read more about that here