Just use following script as below
<meta name="keywords" content="<mt:EntryTags glue=", "><$mt:TagName$></mt:EntryTags>" />
<meta name="description" content="<$MTEntryBody words="30"$>" />
Please be carefully when you change your template file.
Follow me.
Login your Movabletype 4
Desing > Templates
Open Entry Template
Find
<title><$mt:EntryTitle encode_html="1"$> - <$mt:BlogName encode_html="1"$></title>
Add the code after it.
Rebuild your entry archives.
Done.
It is one of the most basic Search Engine Optimize methods. It make your entry page more Search Engine friendly.

Hi David,
Thank you for this post. I've been checking the MT site and the forums but I haven't found a clear answer to MT meta data until this post. I was hoping you might have some additional insight.
I'm pretty new to MT, so I hope you can bear with me.
So I followed the procedure above, but when I navigated to my entry template, I did not find a (title) tag. I believe the title is being generated from the snippet below.
(MTSetVarBlock) name="title")($MTGetVar name="page_title"$) - ($MTBlogName encode_html="1"$-(/MTSetVarBlock)
I tried to insert your script after this and in a few other locations, but it didn't work anyway I tried. The meta data did not show, and worse, it would totally break the page. I wasn't surprised since obviously my template didn't line up with your script. With this in mind, can you provide any advise as to how to get meta data on my entry pages? Can i take out the (MTSetVarBlock) and replace with alternative meta data code?
Also, if I put meta data on the entry pages, can I also have meta data i the header. Previously I had meta data in the header, which was fine if you were on the homepage, but the same meta data would show on all pages. If I put meta in the entry pages, can I also have meta on the home page?
Sorry for the long post. Thanks for your patience. I know these are probably some pretty basic questions.
Mike Sullivan
Warehouse Weekends
You can put the code of my post right before
The reason why you can not found the title is different template. Your template may different.
Put it in header is OK, but do not cache it. Other wise all pages will have same meta.