This is an older theme, it doesn't support the built in WordPress menu feature. So we'll have to make adjustments manually.
Open up header.php and look for this line,
<?php wp_list_pages('title_li=&depth=1&exclude='); ?>
We can do this one of two ways:
1. Note the part that says exclude= ? This is where you can exclude certain pages from appearing in the menu. You'll need the numerical IDs of those pages which you can get by clicking on Pages in the menu on the left side of the page then hovering over each title. As you do so look at the browser status bar at the bottom of the page. You'll see something like,
1550 is the numerical ID of that particular page.
With that in mind simply insert 1550 into that snippet above from header.php. Separate multiple IDs by a comma. For example, let's assume you have 5 pages you want excluded from the nav menu. That line of code might then look something like,
<?php wp_list_pages('title_li=&depth=1&exclude=45,98,250,1550,1700'); ?>
2. Another way to do it is use include rather than exclude. This might be easier if you have a whole bunch of pages you want to exclude. Go back to header.php and look for that line again. Change exclude to include then insert the numerical ID of the page you want included.
For example, you have 20 pages but only want 3 of them showing up in your menu. The line of code might look something like this,
<?php wp_list_pages('title_li=&depth=1&include=8,15,60'); ?>
Notice how we changed exclude to include? Rather than using exclude and listing 17 pages we can use include and list 3. It will only include those 3.