For the drop down menu (with regular mods), it'll only show if they're assigned to a particular forum - if you want your mods to moderate all of the forums, set them as super mods. If you do that, they should be able to see the Mod CP link without any changes.
On the other hand, if you want your mods to only be responsible for just a few of the forums, assign them to the moderator usergroup, then manually add them as mods of the forums you want them to moderate. In the "footer" template, edit this bit of code:
Code:
<if condition="$show['modcplink']"><a href="$modcpdir/index.php$session[sessionurl_q]">$vbphrase[mod]</a></if>
And change it to:
Code:
<if condition="is_member_of($bbuserinfo, 6, 5, 7)"><a href="$modcpdir/index.php$session[sessionurl_q]">$vbphrase[mod] CP</a></if>
The 6, 5 & 7 refer to the usergroups that you wish to see the Mod CP link; 6 (admins), 5 (super mods) & 7 (mods). Change the numbers as you wish to match the usergroup id that can be found on the Usergroup Manager page.