With this modification you'll add a Login Box to the right side of your forum.
Go to: ACP > Templates > Header Templates >
header_welcomeblock_guest > and find;
<span id="quick_login">{$lang->welcome_guest} (<a href="{$mybb->settings['bburl']}/member.php?action=login" onclick="MyBB.quickLogin(); return false;">{$lang->welcome_login}</a> — <a href="{$mybb->settings['bburl']}/member.php?action=register">{$lang->welcome_register}</a>)</span>
and add the following code just after that;
<div style="float: right; width:633; height:70">
<form action="member.php" method="POST">
<input type="hidden" name="action" value="do_login">
<input type="hidden" name="url" value="{$url}">
<table cellspacing="0" cellpadding="0" border="0" width="50%" align="center">
<tr>
<td class="tborder">
<table border="0" cellspacing="{$theme[borderwidth]}" cellpadding="{$theme[tablespace]}" width="100%">
<tr>
<td class="thead" colspan="2"><span class="smalltext">
<strong>{$lang->login}</strong>
</span>
</td>
</tr>
<tr>
<td class="trow1">
<strong>{$lang->username}</strong>
</td>
<td class="trow1">
<input type="text" name="username">
</td>
</tr>
<tr>
<td class="trow2">
<strong>{$lang->password}</strong>
</td>
<td class="trow2">
<input type="password" name="password">
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td align="right"><input type="submit" value="{$lang->login}"></td>
</tr>
</table>
</form>
</div>