Click on the gear icon in the upper right hand corner. 

Click the new button to create a new currency.

Select the appropriate currency from the drop down menu. 

--None--
AED - UAE Dirham
AFN - Afghanistan Afghani (New)
ALL - Albanian Lek
AMD - Armenian Dram
ANG - Neth Antilles Guilder
AOA - Angola Kwanza
ARS - Argentine Peso
AUD - Australian Dollar
AWG - Aruba Florin
AZN - Azerbaijan Manat
BAM - Convertible Marks
BBD - Barbados Dollar
BDT - Bangladesh Taka
BGN - Bulgarian Lev
BHD - Bahraini Dinar
BIF - Burundi Franc
BMD - Bermuda Dollar
BND - Brunei Dollar
BOB - Bolivian Boliviano
BOV - Bolivia Mvdol
BRB - Brazilian Cruzeiro (old)
BRL - Brazilian Real
BSD - Bahamian Dollar
BTN - Bhutan Ngultrum
BWP - Botswana Pula
BYN - Belarusian Ruble
BYR - Belarusian Ruble
BZD - Belize Dollar
CAD - Canadian Dollar
CDF - Franc Congolais
CHF - Swiss Franc
CLF - Unidades de fomento
CLP - Chilean Peso
CNH - Chinese Yuan (Offshore)
CNY - Chinese Yuan
COP - Colombian Peso
CRC - Costa Rica Colon
CUC - Cuban Peso Convertible
CUP - Cuban Peso
CVE - Cape Verde Escudo
CZK - Czech Koruna
DJF - Dijibouti Franc
DKK - Danish Krone
DOP - Dominican Peso
DZD - Algerian Dinar
EEK - Estonian Kroon
EGP - Egyptian Pound
ERN - Eritrea Nakfa
ETB - Ethiopian Birr
EUR - Euro
FJD - Fiji Dollar
FKP - Falkland Islands Pound
GBP - British Pound
GEL - Georgia Lari
GHS - Ghanaian Cedi
GIP - Gibraltar Pound
GMD - Gambian Dalasi
GNF - Guinean Franc
GTQ - Guatemala Quetzal
GYD - Guyana Dollar
HKD - Hong Kong Dollar
HNL - Honduras Lempira
HRD - Croatian Dinar (Old)
HRK - Kuna
HTG - Haiti Gourde
HUF - Hungarian Forint
IDR - Indonesian Rupiah
ILS - Israeli Shekel
INR - Indian Rupee
IQD - Iraqi Dinar
IRR - Iranian Rial
ISK - Iceland Krona
JMD - Jamaican Dollar
JOD - Jordanian Dinar
JPY - Japanese Yen
KES - Kenyan Shilling
KGS - Kyrgyzstan Som
KHR - Cambodia Riel
KMF - Comorian Franc
KPW - North Korean Won
KRW - Korean Won
KWD - Kuwaiti Dinar
KYD - Cayman Islands Dollar
KZT - Kazakhstan Tenge
LAK - Lao Kip
LBP - Lebanese Pound
LKR - Sri Lanka Rupee
LRD - Liberian Dollar
LSL - Lesotho Loti
LYD - Libyan Dinar
MAD - Moroccan Dirham
MDL - Moldovan Leu
MGA - Malagasy Ariary
MKD - Macedonian Denar
MMK - Myanmar Kyat
MNT - Mongoli

Enter the conversion rate.  As of this writing the conversion rate was 0.94 Euros to the US Dollar. 

Click "New" to create a new "Opportunity". 

Search for the account name associated with the Opportunity. 

Enter the amount associated with the Opportunity. 

Click the "*Close Date" field and select the "Close Date" for the Opportunity. 

I selected "Closed Won" for this example. 

Note the amount, USD 1,000.00. But this is actually an Opportunity that came from Europe and should have been recorded at a Euro price. 

Note that the field value now shows the Opportunity in both USD  and the relevant EURO "Amount

EUR 1,000.00 (USD 1,063.83)"

Tip! This is NOT the best method for handling multiple currencies.  It can get very complicated very quickly especially if you are dealing with many currencies.  The values are static and you would need to have someone manage the rates frequently.   If real time conversion rates are a necessity, you may want to research solutions in the Salesforce App Exchange. 

How to Create and Edit Opportunities with Multiple Currencies