Javascript – Cannot find module formidable – Node.js

I'm starting to develop with node.j,I meet an issue regarding the using of the module 'formidable'.

I have this error:

Error: Cannot find module 'formidable'

Here is the module list installed using 'npm ls installed' :

├─┬ express@2.5.9 
│ ├── connect@1.8.7 
│ ├── mime@1.2.4 
│ ├── mkdirp@0.3.0 
│ └── qs@0.4.2 
├── formidable@1.0.9 
├─┬ node-inspector@0.1.10 
│ ├── paperboy@0.0.3 
│ └─┬ socket.io@0.8.7 
│   ├── policyfile@0.0.4 
│   ├── redis@0.6.7 
│   └─┬ socket.io-client@0.8.7 
│     ├── uglify-js@1.0.6 
│     ├── websocket-client@1.0.0 
│     └── xmlhttprequest@1.2.2 
├─┬ npm@1.1.21 
│ ├── abbrev@1.0.3 
│ ├── archy@0.0.2 
│ ├── block-stream@0.0.5 
│ ├── chownr@0.0.1 
│ ├── fstream@0.1.18 
│ ├─┬ fstream-npm@0.0.6 
│ │ └── fstream-ignore@0.0.5 
│ ├── graceful-fs@1.1.8 
│ ├── inherits@1.0.0 
│ ├── ini@1.0.2 
│ ├── lru-cache@1.0.5 
│ ├── minimatch@0.2.2 
│ ├── mkdirp@0.3.0 
│ ├─┬ node-gyp@0.4.1 
│ │ ├── ansi@0.0.4 
│ │ └── glob@3.1.9 
│ ├── node-uuid@1.3.3 
│ ├── nopt@1.0.10 
│ ├── proto-list@1.0.0 
│ ├── read@0.0.2 
│ ├── request@2.9.153 
│ ├── rimraf@2.0.1 
│ ├── semver@1.0.13 
│ ├── slide@1.1.3 
│ ├── tar@0.1.13 
│ ├── uid-number@0.0.3 
│ └── which@1.0.5 
└─┬ socket.io@0.9.6 
  ├── policyfile@0.0.4 
  ├── redis@0.6.7 
  └─┬ socket.io-client@0.9.6 
    ├─┬ active-x-obfuscator@0.0.1 
    │ └── zeparser@0.0.5 
    ├── uglify-js@1.2.5 
    ├─┬ ws@0.4.14 
    │ ├── commander@0.5.2 
    │ └── options@0.0.3 
    └── xmlhttprequest@1.2.2 

I add that it is the only module who generate this error.

Also, I don't really understand the way are encapsulated some module, it appears that npm is installing the module directly in the directory I'm using the module installation command, and I notice that formidable has been installed in the express/connect/ module on its first installation.

Can you give me more information about the module installation tree.
Thank for your replies

Cheers