On previous OS X releases, I could open Disk Utility and choose file->NFS Mounts. I cannot find anything similar in Mavericks. Is there any GUI for mounting NFS, or must it all be done via the command line?
Unfortunately the NFS GUI tools have been removed from disk utility in Mavericks. However the NFS manager tool ( www.bresink.com/osx/NFSManager.html) can replace that functionality and provide even more fine grained control than disk utility's implementation did. If you want this NFS mount to remain mounted persistently you can also use automounter which is a CLI only tool.
A late answer, for posterity and the desperate searcher.
The GUI has been moved more or less to Directory Utility, which can be found in /Volumes/[name of your harddrive]/System/Library/CoreServices/Applications. Use -shift-G if you are lost to find your way.
Open Directory Utility and click on the Directory Editor (third) icon. Select "Mounts" from the dropdown menu Viewing (Node dropdown menu set to "/Local/Default") - if editing is locked, click the lock and enter an admin password. Click the + icon in the lower left corner of the pane to start a new entry. Set "record name" to the server directory that you want connect to: [IP of server]:/[level1]/[level2] ….
Click the + icon in the right half of the pane, to add (and save) the values for VFSLinkDir (full path to the mountpoint, that is, where the remote directory should be inserted), VFSOpts (I am using resvport,nolocks,locallocks,intr,soft,wsize=32768,rsize=32768,nosuid) and VFSType (nfs). The values for AppleMetaNodeLocation and GeneratedUID are generated automatically.
Restart your computer.
Note added: After changing the NAS system I noticed that the above instructions worked with a Seagate BlackArmor NAS, but not with a WD MyCloud Mirror NAS. The WD NAS can be mounted as a NFS share under OS X 10.6.8 without problem. For a working connection with the WD NAS I used the following options: resvport,nolocks,locallocks,nosuid,rw,no_root_squash,no_subtree_check,soft,intr,rsize=8192,wsize=8192,timeo=900,retrans=3,proto=tcp,all_squash (I copied these from another site, maybe not all of them are required).