I ran into this problem today, and I've come up with a few solutions. The root cause is that the default folder that Nexus Mod Manager wants to install into is not typically writable by normal users. They want you to install into a subfolder of C:\, and if you don't have administrator rights, chances are you can't create the "Games" folder you need (C:\Games).
You've got a few options:
Run the program as administrator. This way, it will have permissions to write to the root of C:\, and then you can create the files/folders there. Just right click, and choose the "Run as Administrator" option.
Get write permission on C:\Games. You can do this if you have administrator rights by:
- Open your C:\ in Windows Explorer
- If the "Games" folder does not exist, right click, choose "New Folder" and name it "Games"
- Right click on the "Games" folder, and choose "Properties"
- Click the "Security" tab
- Click the "Edit" button
- Click the "Add" button
- Type your login name (if you log in as "bobsmith" then you'd type "bobsmith" here)
- Click "Check Names" and if the name becomes underlined, you did it right! Click OK. If not, double check your login name from step 7.
- On the bottom half of the "Permissions" dialog, click "Full Control" for the user you just added.
- Click OK to dismiss this dialog, and then click OK on the properties dialog.
Now you can write to C:\Games, which ought to make NMM happy.
Put your Nexus Mod Manager files somewhere else. If you install to some subdirectory of your home directory, you won't have to worry about permissions and so forth. When the Mod Manager asks you where you want to save the mod files, pick someplace under C:\Users(your user name)\ or C:\Documents and Settings(your user name), depending on the version of Windows you're using. You could potentially put this in your Documents folder, but I have "projects" folder I created in my home directory, which works well for me.
You do not want to set these directories to something under C:\Program Files or in the Steam folder, etc. This is a Bad Idea.
If you've already selected the default mod directories, you won't be asked if you want to change them when you get this error. If that's the case, you can edit or delete the config file so that you can pick a new directory.
The config file for Nexus Mod Manager is stored in %LOCALAPPDATA%\Black_Tree_Gaming - you can copy and paste this into the address bar of Windows Explorer to go there, or paste it into the "Run" or "Search" dialog on the Start menu. Alternatively, from your user's home directory, look for "AppData" and then "Local" and then "Black_Tree_Gaming"
The name of the directories in this folder depend on your version of NMM, but the file you're looking for is user.config. You can delete this file, but you'll lose any other NMM settings (including the location of any games that were scanned previously). You can also edit it with a text editor.
Look for a block that looks like:
<setting name="ModFolder" serializeAs="Xml">
<string>C:\Games\Nexus Mod Manager\Skyrim\Mods</string>
<setting name="InstallInfoFolder" serializeAs="Xml">
<string>C:\Games\Nexus Mod Manager\Skyrim\Install Info</string>
The directories here are the ones you'd want to change to some folder you can write to.